Simple answer: There is no one best.
Despite what people on this and practically any other audio forum out there say, there is no one best-in-the-world coax, component set, subwoofer or amplifier. Some will say that JL is the best, some jump the Rockford and Kicker bandwagons, and you'll have fanboys out there for practically any brand that you can think of from Alphasonic to Zapco. Some of these people are blind followers that believe anything that they are told by marketers, some believe that whatever they happen to own is the best, some will tell you that a certain brand is the best because that brand sponsors them and so on, but none of them can scientifically prove that their reasoning this fact rather than mere opinion.
Full Range Speakers
When it comes to full range speakers, ie coaxes and component sets, you'll find everything from $10 sets that you've never heard of to sets in the thousands that ship in pretty wooden crates. What makes any of these better than the rest? It comes down to build quality, materials, engineering, research and innovation. People will describe the sound of speakers with just about any descriptor that you can imagine from the usual "warm", "bright", and "harsh" to the more unique "silky", "wet", "alive" and other terms that make you wonder how hard they had to think to come up with that term to describe a speaker's sound quality.
What really matters is how the speaker sounds to you! If you take people's word for everything in life you'll miss out on one of the main points of living. Go listen to speakers in your local shops and stores. Sure, they'll sound different when you put them into a car, but at least you can get some idea of what you're looking for. Any shop worth its mettle will have a demo car on hand for you to listen to or a customer's car that may be listenable. If you want to take things to the next level, check out local car shows or IASCA or USAC shows, or for you Europeans EMMA shows and you'll find plenty of people that'll gladly let you hear their cars. This will give you a reference of what speakers that are properly installed can sound like, as the install is more important than the speakers themselves.
What sounds good to your ears will be unique to you and what you deem as "good" sound quality. Obviously a person who listens to nothing but rap will have a totally different definition of "good" sound than a person who loves his heavy metal or Mozart. Also important: Your hearing. An adult male typically can't hear above 16000hz, and depending upon life experiences may have significant hearing damage. Therefore if you and a friend listen to the same car and music, you're likely to have totally different impressions of its sound quality. If you're looking for realism and a sound quality system, you need a good reference, and live, non-amplified music is the best that you can get. I'll guarantee that if you go and sit at a jazz club one night and really listen to the music you'll have a new impression of what music should sound like.
Subs
If it weren't for subwoofer debates, 50% of the car audio posts on the web wouldn't exist. People argue about subs like cats and dogs fighting. Much of the above material applies to subwoofers, from the engineering and build being of importance to the guys with blind brand loyalty spouting off their mouths. I can say this about subwoofers: The install is more important than the subwoofer itself. You can take a subwoofer and place it into different enclosures and? you'll hear totally different characteristics in the output. A sub system (box+sub+amp) can be optimized for sound quality, sheer output or just about anything in between. There is no one best enclosure or one best sub, plain and simple.
Amplifiers
Yet another source of pure stupidity here. I've had people tell me that their amp was the best in the world for reasons beyond my comprehension in some cases. People on forums will go on and on and on about how the sound quality of their amps really makes their system have a "warm" sound, others will create more unusual qualities that their amps somehow create.
The fact of the matter is that an amplifier is meant to be nothing more than a straight wire with gain. It takes its input and increases its magnitude, both the input and output should have the same waveform. If an amp is adding some magical properties to your sound, it's not doing its defined job.
Buy an amp that has a reputation for being well built and is reputable, and most importantly, fits your needs as far as output power and size are concerned. There are dozens of amps ranging from dirt cheap swapmeet amps to mid priced pieces up to amps in the thousands that will fit the bill, but always remember that you usually get what you pay for when it comes to amps. If you buy that $50 amp that has "5000Watts Max Power" proudly emblazoned across the top of its heatsink, don't expect it to make that much power, or any at all if you're really unlucky.
This is all just my opinion and what I tell people that I talk to in person that ask me audio questions, so your thoughts on these subjects will of course vary, and that's fine. If they do, post what you're thinking!?
Despite what people on this and practically any other audio forum out there say, there is no one best-in-the-world coax, component set, subwoofer or amplifier. Some will say that JL is the best, some jump the Rockford and Kicker bandwagons, and you'll have fanboys out there for practically any brand that you can think of from Alphasonic to Zapco. Some of these people are blind followers that believe anything that they are told by marketers, some believe that whatever they happen to own is the best, some will tell you that a certain brand is the best because that brand sponsors them and so on, but none of them can scientifically prove that their reasoning this fact rather than mere opinion.
Full Range Speakers
When it comes to full range speakers, ie coaxes and component sets, you'll find everything from $10 sets that you've never heard of to sets in the thousands that ship in pretty wooden crates. What makes any of these better than the rest? It comes down to build quality, materials, engineering, research and innovation. People will describe the sound of speakers with just about any descriptor that you can imagine from the usual "warm", "bright", and "harsh" to the more unique "silky", "wet", "alive" and other terms that make you wonder how hard they had to think to come up with that term to describe a speaker's sound quality.
What really matters is how the speaker sounds to you! If you take people's word for everything in life you'll miss out on one of the main points of living. Go listen to speakers in your local shops and stores. Sure, they'll sound different when you put them into a car, but at least you can get some idea of what you're looking for. Any shop worth its mettle will have a demo car on hand for you to listen to or a customer's car that may be listenable. If you want to take things to the next level, check out local car shows or IASCA or USAC shows, or for you Europeans EMMA shows and you'll find plenty of people that'll gladly let you hear their cars. This will give you a reference of what speakers that are properly installed can sound like, as the install is more important than the speakers themselves.
What sounds good to your ears will be unique to you and what you deem as "good" sound quality. Obviously a person who listens to nothing but rap will have a totally different definition of "good" sound than a person who loves his heavy metal or Mozart. Also important: Your hearing. An adult male typically can't hear above 16000hz, and depending upon life experiences may have significant hearing damage. Therefore if you and a friend listen to the same car and music, you're likely to have totally different impressions of its sound quality. If you're looking for realism and a sound quality system, you need a good reference, and live, non-amplified music is the best that you can get. I'll guarantee that if you go and sit at a jazz club one night and really listen to the music you'll have a new impression of what music should sound like.
Subs
If it weren't for subwoofer debates, 50% of the car audio posts on the web wouldn't exist. People argue about subs like cats and dogs fighting. Much of the above material applies to subwoofers, from the engineering and build being of importance to the guys with blind brand loyalty spouting off their mouths. I can say this about subwoofers: The install is more important than the subwoofer itself. You can take a subwoofer and place it into different enclosures and? you'll hear totally different characteristics in the output. A sub system (box+sub+amp) can be optimized for sound quality, sheer output or just about anything in between. There is no one best enclosure or one best sub, plain and simple.
Amplifiers
Yet another source of pure stupidity here. I've had people tell me that their amp was the best in the world for reasons beyond my comprehension in some cases. People on forums will go on and on and on about how the sound quality of their amps really makes their system have a "warm" sound, others will create more unusual qualities that their amps somehow create.
The fact of the matter is that an amplifier is meant to be nothing more than a straight wire with gain. It takes its input and increases its magnitude, both the input and output should have the same waveform. If an amp is adding some magical properties to your sound, it's not doing its defined job.
Buy an amp that has a reputation for being well built and is reputable, and most importantly, fits your needs as far as output power and size are concerned. There are dozens of amps ranging from dirt cheap swapmeet amps to mid priced pieces up to amps in the thousands that will fit the bill, but always remember that you usually get what you pay for when it comes to amps. If you buy that $50 amp that has "5000Watts Max Power" proudly emblazoned across the top of its heatsink, don't expect it to make that much power, or any at all if you're really unlucky.
This is all just my opinion and what I tell people that I talk to in person that ask me audio questions, so your thoughts on these subjects will of course vary, and that's fine. If they do, post what you're thinking!?