I'd chose Dave. I wasn't a big fan of him the first time I heard him in The Chipmunk Adventure as I thought he was a little annoying at the time. After watching many of the episodes, I think he's a great father figure that somehow puts up with Alvin. Ian was more or less selfish and isn't a good taste. I do like the way he reformed in Chipwrecked though.
Ick. Ian? He was such a jerk to the chipmunks, chipettes, and Dave, who was supposed to be his best friend. Ian makes me sick. But Dave, he's GR8! I mean, he found Alvin, Simon, and Theodore in a tree at the mall, took them home in a muffin basket given to him by a friend, and all Ian did come the second movie was torture the chipettes, and in the first movie, he tricked the chipmunks to go on tour with him but ended up locking them in a cage! Dave is actually nice to both the chipmunks and the chipettes, and he makes a great dad, granted it's to a couple of chipmunks, but hey they make a great family. And although Ian "reformed" in the 3rd movie, come to think Ian would be in the 4th movie, and still be a scumbag jerk. So I think he used Dave, the chipmunks, and chipettes to get his "revenge" on the whole family of a human man and his chipmunks (so more like a guy and his pet chipmunks.) Besides I don't see what Dave did wrong, I mean he's just protecting his pets.
So I'd have to go with Dave. Besides, he got hunky fast in the new Alvin series.
Hmmm.... I'd choose Ian, he's funnier, although Dave is the good one. Also, at the end of the third movie, we saw Ian's heart, so he's not heartless... and I don't mean literally, I mean when he saved Dave.