For the Sonic franchise, while you can technically start anywhere, the best place to start IMO is Sonic the Hedgehog 2. It's a simple, 3-hour game that gets across the main points of the series incredibly well. All the series staples are there, with an easily digestible campaign, being a straightforward, score-based platformer rather than a sprawling story like later games. And while Sonic 1 is there, I feel it hasn't aged nearly as well, and is far less suitable as a first game.