Cheatomorpha is my favorite. It doesn't put down roots into your system, it seems to grow very quickly, and the plant stays green throughout, even the underside that sees very little light.
Cheto is preferred because it doesn't go sexual, but grape and other forms of calurpa (sp) I think are more effective because they grow faster. They can get out of hand and get into the display, but if you have enough herbivores, that shouldn't be a problem. The trick is proper lighting cycles and trimming to keep things from going sexual on you.