Purely due to nostalgia for me, 2 easily. Plus you can get someone to use the P2 controller to totally cheese bosses as Tails!

You're going to want to get both Sonic 3 and Sonic & Knuckles, but I think you should also figure out if you still actually like the old Sonic games. I grew up with them, I considered Sonic 3 & Knuckles my favorite game for quite some time ... now I realize that holding right isn't exactly a thrilling experience.
 
There's also that pesky Spider-Man 3 time paradox theory where the crappiness of Sonic's recent games has somehow traveled back in time, thus creating an alternate time line in which all Sonic games are terrible. 
 
It's a pretty good sale though! :D

 Sonic &  Knuckles for me, its got the most game play and good progression IMO, just don't do the original first, do it last if at all, that is a purely nostalgic experience to me and newer gamers probably wont get the same enjoyment out of it.  

Sonic 2.

By themselves? Sonic 2.
 
Sonic 3 & Knuckles trumps all though.