Good question. Despite what some people would say, there are a ton of really good Sonic games to choose from, and they each bring something unique to the table. I think I'll go with Sonic 3 & Knuckles. Great level design, great use of characters (And a perfect example of why SEGA should at least bring Tails and Knuckles back as fully playable characters) fun power-ups, awesome music, and I actually feel incentive to get the Chaos Emeralds.
But if I were to ignore my sense of judgement and focus on nostalgia, I'd go with Sonic 2 and Sonic Adventure 1. For me, these were the two definitive Sonic games growing up. And Sonic 2 has Casino Night Zone, which is about as 90's as you can get.