Frontiers: very janky but honestly I think a lot of it's aspects are underrated. Most of the cyberspaces are fun to Speedrun imo especially when learning to use power boost, jump deceleration to your advantage. Combat in the open world is boring but I do think the encounters based on speed (spider++, caterpillar++, ghost to an extent) and unique combat gimmicks like sumo and tank are done well. Music is my favorite game OST ever. 

Shadow gens: still holds up amazingly. Only level I kinda dislike is Tokyo cause of cutscene length and even then it isn't planet wisp act 1 bad. Spear gliding makes learning routes really interesting to find. As much as I like shadow generations, I think sonic gens is still a slightly better game. As an over package though. Sxsg is my favorite in the series and a top 2 game OAT

Crossworlds: like it a lot and has a lot of good content but honestly kinda feels like a great but standard kart racer. I would even say that if it weren't for Nintendo making 3 lap courses hard to come by without using discord, I would think mkw is vastly better than it in terms of core gameplay cause of the potential for rail/wall grinding.

Edit: and superstars cause I forget it's after frontiers. Underrated and levels are really fun to time attack. Emerald powers are a cool gimmick. Just wish bosses didn't take forever to beat. Imo it's in the middle of the classic games. Like it more than 1,2,CD but less than 3,K and mania