From a gameplay perspective? 2K11 is king, imo. Was there still some b.s. and rubberbanding a.i.? Yes, but it was minimal in comparison to other games in the series. I never got better games vs the a.i. or human competition than I did in 2k11. Everything felt organic and the players didn't feel like they were stuck in the mud. When I lost, it was because I was outplayed. I also loved those size up moves, particularly Michael's which were scary authentic. Speaking of Mike, I believe this was the first 2k to feature him as playable and to offer so many versions of him. Playing as 80s Mike was different than the '96 version. Really fun game.
My personal favorite however, is easily 2K7. It's nowhere the smoothest or deepest. In fact, compared to the games that followed, it was bare bones, but it's the very first "next gen" title I popped into the system after jumping from the PS2 to the Xbox 360. It was surreal how significant the leap was from a gameplay & technical perspective. I had it on PS2 first, so going from
To
Was
insane.
So many players having signature shots, the sweat that you could actually see build and fly as players minutes increased, and the enhanced court was crazy to me. Of course it isn't impressive 12 years later, but it's probably the biggest leap I've ever experienced. I used to destroy teams w/ the '05 - '06 Kobe.