Shovel Knight is a wonderful game. I'm trying to drum up some love for Serpent in the Staglands.
It hardcore slipped under the radar but is a beautifully crafted love letter to CRPGS of the 90's. Very fun, insane character building options with no classes, and quite easy to speed run. About to submit my run of 41:59. I'd love to see someone else run it since nobody else is.
Dungeon Defenders has eaten up a bit of my time, as has the Binding of Isaac/Binding of Isaac: Rebirth. Plants vs Zombies is slightly older now, and there is a mobile sequel, but playing it on PC just feels better to me. Shovel Knight is a good choice, as is Cave Story+. Granted, only a few of these are decent speedrunning choices (speedrunning a tower defense game would be difficult, I think,) but they are all highly enjoyable to my mind.
Shovel Knight, Binding of Isaac Rebirth, Thomas was Alone, Love, Mark of The Ninja, Risk of Rain, and Transistor are some of my favourites of the top of my head.
I will also say Shovel Knight, especially with the new free DLC. I also can't say enough good things about Axiom Verge, if you like Metroidvania style games it's pretty awesome.
Asking for a game on steam will give you all manner of answers, including plenty from genres you may not be interested in. You should indicate what kind of experience you're looking for - depending on if you want an emphasis on story, strategy, technical skill, or creativity, the games you should be looking into will be completely different.