Beating the final mission of OG Spellforce 2, beating an Undertale genocide run, being my first true bullet hell game, and probably having enough guts to even pull that out.
Also advancing a lot earlier than I thought I could in Age of Empires II. As for now, I can go to Castle Age in 18:30 (even if it was as Huns).
Beating Moon Lord the first time with a strategy that should be counter-intuitive (circling around ML like a madman).
And of course finishing games ranging from hack n' slashes like Diablo, by Action RPGs like Secret of Mana, by shooters, to RTSes and all routes of Undertale, to sandboxes (Starbound n' Terraria), and even Dead Cells I've done.
Being jack of all trades doesn't mean I can't be master of any ^^.