Heavenly is only good for the absolutely amazing views. You have the desert on one side of you, the lake on the other. It is incredible. There are some really long runs and steep tree runs, but it's not really good for snowboarders. However, you can't beat the convenience if you're staying in South Lake. The gondola is right there on the street, just walk from your hotel and hop on. Very, very touristy.
Kirkwood is absolutely the tits. Rarely crowded. I have a t-shirt from there that says "If the mountain crowded, find another run until he's gone." That's really true. Especially on the back side, The Wave (shaped like a giant wave, natch), I've taken top to bottom runs many times with NO ONE nearby. It starts on this massive ridge and ends up with several options through nice wooded trails. Beautiful. But man, if it's windy (which it always seems to be at Kirkwood) it can be crazy up top. Still, best variety of terrain, from insane chutes to big open bowls and deep woods. My favorite place at Tahoe.
I'd suggest looking into a Snow Bomb card. It costs about $60, but usually comes with one pre-paid ticket to somewhere at Tahoe, then pretty steep discounts to the other mountains, plus a couple of "free" tune-ups. If you're there for any length of time, it usually pays for itself.
The Tahoe City area has Homewood...tiny by comparison, but the nicest views of the lake you'll ever see, and it is super quiet. They own Alpine Meadows, and if you stay a couple days (or drive if the road is open) you can get cheap passes that are good for both mountains, and I think even Squaw, which is on the other side of Alpine. There is noplace "bad" at Lake Tahoe. Northstar is super-touristy, but the blacks are fun with good snow. Otherwise, it's like an expensive mall with a mountain attached. Diamond Peak is only worth going to if you have a $10 ticket, which you can actually find if you look around. Tiny. It's advertised as a kids mountain, but it's pretty. If you want to venture out, try Snow Bowl. It's up north, have to drive through the Donner Pass to get there, but it's got some really great terrain, too.