It's hard to think of a favorite, but I would consider Dungeon Keeper 1 & 2. Partly because there's still no modern game which has executed this style of game design as well as these original two games.
My favorite retro game is Super Mario Land 2: 6 Golden Coins. That is because it was the first game I purchased for the Game Boy after I got tired of Tetris. I played it so many times! I can still remember every level. Good times.
My favorite retro game is probably Mother 1 for the Famicom. Specifically the fan translation of the GBA version. It has a cool atmosphere and challenging game play. Plus the story is pretty good for NES.
My hot take about Earthbound is that the official translation kinda sucks tho. I've played with the official translation and the Mother 2: Perfect Edition fan translation and greatly preferred the romhack
Mother 1 is my favorite, it's got such a unique atmosphere. Some of my more recent favorites include the Valis series (bishoujo action platformers), and Monster World IV (action platformer with light RPG elements). I'm big on aesthetics and those games really appealed to me!
Probably https://umoria.org/ I play it often still. It's just so easy to dive into and delivers all the fun tension you'd want but doesn't need you to keep tabs on a story or quests. It's great to dip in for a short run or go for a long time.
Always down to just lose myself in DKC and DKC2. I feel both games are just perfect in all aspects. Gameplay, sound, visuals. True classics.
The 60 FPS version of original Ridge Racer that came on the bonus disc for Ridge Racer Type 4. It's great to pick up for a few races and put down, and the soundtrack is great.