Mine honestly is Sonic the Fighters It's so corny and cartoony. It makes me think of the Looney Tunes which I loved when I was little in addition to Sonic. For my partner, his favorite non main line is 3D Blast.
Sonic the Fighters is a good pick. I would really like to see another attempt at a fighting game with the Sonic cast!
All-Stars Racing Transformed Along with Tails Adventures. A pretty fun and well-designed Metroidvania that gets criminally overlooked for being a Game Gear Tails spin-off with slower (read: more traditional) gameplay, which was deliberate. Plus it had charming sprites and good music.
I'm not sure if sonic Battle is a spin off but that has to be the best by far (in my opinion). The fighting is very fun to pull off, and The plot is some of the best I think the franchise has ever seen.
Sonic Riders (the first one). Getting X ranks is so damn easy you're probably gonna spend most of a race with a full Air Tank... unless you suck at the game. Plus, Sand Ruins is a fun racetrack.
I love flinging myself around in chaotix and it’s probably the spinoff game I’ve played the most. If platformers don’t count then it’s mean bean machine easily. Not saying I’m great at puyo puyo but I’m good enough to enjoy it.
Not only that, but Sonic doing all his attacks being curled in a ball it makes pinball kinda his natural home for a spin-off. Now, a part of me really wants a better version of Sonic Spinball, but the other part of me finds it amusing with all its glitches that can oftenly screw your game altogether. From all the Genesis games I had, this is the one that treats players the worst, and I love it for that.
The aesthetics of Sonic the Fighters are basically unmatched for me. Complete god-tier character models
Sonic Spinball is probably my go-to. Though I also really like (the Wii versions specifically) Mario & Sonic at the Olympic Winter Games and Mario & Sonic at the London 2012 Olympic Games. Those games were both a ton of fun and had fantastic soundtracks and so much great fan service. Kind of a shame the series fell off after those two.
Gonna have to go with the boring answer of All-Stars Racing Transformed. It's just an amazing loveletter to Sega that I can't help but love it. Also helps that it's an amazing game to play, with a phenomenal soundtrack. Wish I liked Team Sonic Racing more as a followup, but I think that's a fun game in its own right. Also wish they patched that game for the online to properly work on PC without frequent disconnects.
This is a pretty interesting game but I think the Power Stone style gameplay would've served better as a home console game with 3d graphics, making it easier to discern depth and allowing for more interesting stages, while the story would've served better in a Sonic (turn-based, action, w/e) JRPG. The fights feel insanely repetitive in the main story, and the the overall progression of upgrading Emerl would've been better in a JRPG imo.
It is impressive and it is pretty fun. But like most Sonic spin-offs it feels like there is a lot of missed potential that could be addressed in a sequel that never comes...
Sonic Riders. I adore the aesthetic, campaign story, etc etc, but the main draw for me is racing against other people via netplay. It's just super fun to hop on Dolphin and try your best to win in Riders, it's got so much depth to the racing that it never gets old. I haven't played it in online proper for like 6 months now, but just typing this makes me want to hop back on.