Plants vs Zombies 1 is great and has SO MANY bonus games you can unlock. Your time spent farming coins is always rewarded. Hint: you'll need close to 1 million coins total to unlock everything, but it's not super hard once you've gotten everything you need in place. You can get over 100,000 coins a day
PvZ2 "It's About Cash" is very VERY aggravating at first, but I have warmed up to it over the past month or so. It was a sickeningly blatant cash-grab when I first started playing it, but somewhere along the way EA realized how badly that was putting people off. You can be successful without ever spending a dime, but it's worth by a long shot to buy the extra seed slot. I've spent about $15 total on it, much of which is wasted. The only plant I bought that I use regularly is the jalapeno. I've beaten all 100 levels and just play Pinata Parties and occasionally Endless Zones until the next world is released. It lacks the magic of the first as well as the variation (night, pool, rooftop, etc)
Reaper by Hexage. CRAZY fun game. The controls are spot on, and you only pay $5 for a full unlock, which is well worth it. Awesome bathroom game you can pick up, play for 15 or 20 minutes, and never forget what you need to do next when you play again. You eventually reach a point where you become all but invincible, but it takes some time to get there. Hexage promised some new content, but I haven't seen it yet. Still a great game.
Other than that, I haven't found a satisfying shmup for tablets at all. Sector Strike is okay, but repetitive and lacking any real boss battles. Most other are either bullet hell, or cash-for-upgrades, neither of which tickle my fancy.
"No I don't want the Ask toolbar! No I don't want Bing as my default search! No I don't want to make Chrome my default browser!"
"Good grief, man! WHAT are you trying to install on that poor computer?"