Back in the day, Sonic 2, Sonic 3, and Sonic & Knuckles. I find the series can be a hard sell to people who didn’t grow up in that era though. Sonic 1 is a classic too, but doesn’t hold up as well as the others, imo.

Streets of Rage is another solid trilogy. I think it peaked at SoR2, but the other games are fun too.

If you’re just looking to play games and not buy them, Crusader of Centy is a lot of fun. A bit of a unique Zelda-like game where instead of new items, you get new animal followers!

The Shining Force duology is great, too. It was probably one of the first SRPGs to be released in North America!

And obligatory name drop for Star Control. The sequel is so much more fleshed out, but this one is a tactical strategy game where you have space ship fights when you encounter the enemy. There are two sides, each with ~7 unique and distinct ships!

Other than those…check out Sega Lord X on YouTube! He always talks about Shinobi, the arcade ports, and other awesome Sega games!