Keyboard/Mouse games (some have compatibility for controller but why):
Age of Wonders 3 (If you have friends to play with. The story is kind of meh, and playing by yourself against AI can get pretty stale. Recommended that you turn auto-battles off except between AI. Play your battles, it's so much more fun and rewarding)
Chroma Squad
Gigantic (would play it more if it wasn't such a team reliant game. hate doing well and still having a landslide defeat, or barely trying and having a landslide victory. still good though.)
Serious Sam HD: The First Encounter
Serious Sam HD: The Second Encounter
serious sam 2 sucksSerious Sam 3: BFE
Styx: Master of Shadows
Terraria
Some games that are fine with either keyboard/mouse or controller, or even a steam controller:
Valdis Story: Abyssal City
Rogue Legacy
Salt and Sanctuary
Some games that are highly recommended that you use a controller for:
Beat Hazard
Spacecats with Lasers : The Outerspace
Thumper (Just as good with a steam controller)
A game that's poorly ported and is practically impossible to play with Keyboard/mouse/Steam controller almost entirely due to how the controls work:
NieR:Automata
Lethal League, if you have a controller / think you'd enjoy versus-style fighting games
I loved this game even though I was pretty bad at it. Not the absolute worst, there were several nobodies in the QM that I could trash. But out of all the known players, I was pretty far down on the list. It became a thing for the tourney chat to wonder if I was going to get a single win. I was the only person who would consistently enter and lose, every single time. Except that one time I won a round against some new guy.