Dead trigger is free and amazing. Seriously, I had played many shooters on my iphone and ipad and hated all of them... this one was great, couldn't stop playing! So pissed because I lost all my progress when I got my replacement device (worth it tho lol). GET DEAD TRIGGER.
I also like where is my water/ perry (2 different apps but the same game w different levels). I had seen them on top of the app store and google play for a while and decided to give it a shot, loved the Perry one so much (and beat it) that I bought the water one (the alligator one). It's a fun game. Nothing deep or intense obviously, but my kinda tablet game. If you like simple tablet games, get one (or both).
Those 2/3 get the most playing time, I got plenty of other games (mostly free) that just sit there that I wouldn't recommend. GTA 3 is something I might buy that looks fun. I also just downloaded Open Sea, but haven't had time to play.