I think you would have a very different opinion if you grow up in the 90s and started with Road & Track's NFS
Not saying that NFSU isn't great btw. I played the hell out of that game and NFSU2 back in the day. But the NFS series as a whole has 3 very distinct eras and it's impossible to say one game is the absolute best when games from different eras are incomparable.
Exotic Era: R&T NFS - NFS HP2 (best game probably NFS3 HP or NFS PU)
Custom Era: NFSU - NFS Undercover (best game either NFSU2 or NFS MW)
Identity Crisis Era: NFS Shift - NFS Rivals (best game arguably NFS HP 2010)