I have a 12” Xtant X1244 (kinda rare these days) in a 3.8 CF ported box. It’s tuned to 28 Hz, running at 900 watts RMS, and will give a 12” W7 a run for it’s money in terms of output and sound quality. I can’t say Xtant is the “best”, it depends what you want. The Xtant leans toward SQ, but can have some sick output on a big enough amp.
Orion HCCAs are good solid SPL subs with huge 4” (I think) voice coils that dissipate heat quite well. HCCAs will take several thousand watts before they complain, and are quite efficient considering their power rating. Despite being billed as SPL subs, HCCAs generally have a fairly realistic sound.
Both of the above are solid, competition ready, and can be had for a reasonable price. The best sub in the world? Probably an obscure brand with a huge price tag. I’m not going to guess.