Titan or one of the other skins on iPoker work well and have good traffic. The multi-tabling on ipoker works fairly well.
Party I haven't played for a long time and I didn't like their software much, but when the dust settles I would think they are going to be one of the first back into the US market.
Littlewoods or 888
.com are both worth looking at, multi-tabling was a bit difficult and their software need some improvement but the micro cash tables are very fishy
Merge is worth a look as it is now he largest network accepting US players so the traffic should increase.
I also play at Victor Chandler on the Entraction network, great micro cash tables as there are loads of very bad players on there. Down side is their software is really bad for multi-tabling. On FT and PS the tables wanting attention wait for you to finish the action on the focused table, on entraction they just jump in front. So you can be making a decision and suddenly your table disappears behind 2 or 3 others it's really annoying.
Hope some of that helps.