I have favorite games for a bit and then get tired of them.
My favorite iPhone games have been:
Peggle - I love this game. I still play it fairly often just for the challenge levels.
Plants vs. Zombies - it's a bit too easy but it's a fun way to pass the time.
Tilt to Live - great game. Good music, slick execution.
Doodle Jump - a fun one for the kids, but I like it sometimes too. Especially when my oldest daughter gets close to my high score and I have to move the bar up a bit.
iNethack - a game about neither hacking nor the internet, it's a turn-based randomized dungeon-crawling RPG. I've been playing Nethack in it's incarnations since about 1988. The iPhone version is free and just as ridiculously hard as the PC version and a great way for me to waste a ridiculous amount of time.
Wurdle - like Boogle
Bookworm - like Boogle
Geodefense and GD Swarm - Tower Defense - I like GD Swarm better but they are both great.
Angry Birds - lots of fun. I like that they keep adding levels for free.
I liked
Zombie Smash, although I thought that I'd get some repetitive stress injury on my thumbs if I didn't stop playing.