Obviously there are a lot of bands who are really in to what they do without beaming with a big grin the whole time, and plenty of bands for whom that would totally ruin the atmosphere anyway. But sometimes I like seeing bands who are clearly just loving playing their songs, and I can't actually remember that many that I've seen. Off the top of my head, there's:

Torche
Fang Island
Rival Schools

...and that's all I can remember. Maybe there's more, I dunno. Who have you seen that just seem proper happy being on stage?