JL is a really nice sounding sub. Keep in mind when you say the "best" you have to ask yourself, the best for what? There are subs on the market that are designed for excursion.. and there are other brands that are designed mainly for SQ.. and won't hit as hard so when you say below $600 you have to ask yourself... what do I want?
Do I want a sub that will blow my windows out, or do I want a brand that is designed for optimal SQ, but at the same time won't come anywhere near busting the windows out. Basically what I"m saying is that there is no "Best" brand out there because each company has different goals.