I'm currently doing Castlestorm and Plants Vs. Zombies 2.
Castlestorm combines fun elements of Angry Birds (Angry Spear?) with tower defense, medieval fantasy elements, in a 2d side view environment.
I've always been a fan of the first PvZ. They kept the humor, game balance, and creativity from the original that I liked.
You really don't need to spend $$ on either.
However, I would like to suggest you spend $3 or $5 for the VIP Gamer IAP to get gems and coins every week, double XP, and remove ads during gameplay.
For PvZ2, spend $4 to $10 to max out all gameplay boosts. I ended up NOT spending any $$ of these, and I'm treating the regular game as if I had 1 less seed slot, 1 less plant food slot, and 2 less of the other upgrades.