which 15 sub is best?

Answers to questions like these will lead to people giving thier opinions but ...that will not nessecarily mean it  is "best" for you. What sounds good to one person may not to another and there are other considerations such as the subwoofers primary purpose..ie: SPL, sound quailty, type of music etc..

Try to listen to different brands and models and see which one you think is best..that will be the best for you. However reliability is also a consideration..but I find plenty of opinion there too which can make your decision tricky by strictly basing it on someone elses experience. In other words I often find one person will tell me a particular sub is crap...and then someone else will say the same sub has lasted them for years and still kicks butt.

As a rule of thumb and assuming the ratings are honest on the product if you use a woofer in which its RMS rating is equal to or greater then the amps RMS rating you should be safe. This of course assumes a proper enclosure for the sub (sealed/vented/bandpass), proper air space, well constructed and the amp gains set correctly as well. Most of us know that improper gain settings is a major cause of blown subs. A common misconception is that cranking the gains makes the amp more powerful. If this was true why would the amp manufacturers even put a gain control? Keep that in mind.

Best of luck and let us know what you decided on.