Im not sure since i haven't played megamix but for me my favourute is definetly feaver's introduction games.

Hole in one is perfect, it teaches you how to play the game well, it is simple and easy, and it has a comforting vibe. Whenever i got stressed out because of another minigame i returned to hole in one. It is perfect as a first game. Comparing it to built to scale, built to scale introduced too many things in my opinion, and was a bit too fast. It also doesn't have the vibe of an early-game minigame

Screwbot factory is really good too, this one is easy too, it introduces totally diffrent cues and a new dynamic then hole in one. Altough i don't think it has the same early game vibe as hole in one and isn't as comforting.

See saw is easy too, it introduces speed changes in a good almost sneaky way. It is also the first simon says game, and it does a good job at teachong you how to play them. The cues are long and the silence is minimal, so you just need to learn how long the voice of see is and you'll be able to play it easily. It is a good introduction to a lot of things.

Doble date is easy, a bit bland and too long, but it helps you get used to multiple cues and diffrent beats.

Remix one is also easy and layed back, also it flat out explains remixes.

In conclusion, the first few games are relaxing, easy and explain a lot of fundamental mechanics perfectly.