Everybody will have their own opinion. IMO
Free Game - Dredd vs Zombies :Why? No/Very few bugs, Superb graphics, I feel I have a GOD given right to kill zombies, lots of levels to waste time.
Paid Game -Asphalt 7 : Why? Coz it runs very well on any windows phone, has no issues at all (AFAIK) , superb gameplay, superb graphics, lots of levels, multiplayer, tons of cars.