Chef Chen definitely has my vote. IT's good stuff. A very notable alternative if you get there and it's packed and don't feel like waiting.... there's KAJU TOFU, 2 or 3 doors down. I love the tofu there and their sides as well, but be forewarned their SERVICE sucks ass! But I think it's one...
Try Gen Kai. It's new in Irvine, on Jeffrey Road and Irvine Center Drive in the same plaza as the Thai Spice. They have a restaurant in Corona Del Mar as well.