On being indecisive...

Definitely get Rocket League. It's loads of fun, even if you're just playing by yourself.
Edit: Just realized you said it has to be 50% off or more and this is only 40% but I still think it's a great deal.

Tomb Raider for $5? Hell yeah

Witcher 3 worth every penny full price, and then some.

Rocket League is addictive as hell. I was skeptical myself.

And Undertale is a nice experience.

For your wishlisted games the deals are pretty shocking imo.

Get Stacking if you really want it, given it's 90%
Witcher 3 is something I'm tempted to get, so maybe you should do too? (If you own the first two, get the trilogy pack for discount)

Found Stacking to be a great experience if not perfect mechanically, something to play through and never touch again. At that price it's a bargain imo. Can't speak to most of the others, Bioshock is good if not the revelation some people claim, can't really go wrong at that price.

Get
Double Fine's; Stacking, Grim Fandango Remastered and Psychonauts
Telltale Game's; The Walking Dead (and 400 days dlc), Walking Dead Season 2, and The Wolf Among Us
All awesome adventure games.

Stacking is on my list.... hmm... wait what........ 80% off the Double Fine Bundle 2015. Take my money. That entire bundle is on my wish list.

Go for The Witcher 3, I'm 99% sure that you won't regret it. If you want to play FF VI, just download a gba emulator and play the gba version.

Witcher or Stardew Valley! One piece of advice, don't buy bundled games though, better trade for them! I could give you Murdered Soul Suspect for 1euro, not 7.50 and I think a lot of people has it from humble bundle:)