I'll start. I think the Rattlesnake is absolutely incredible for mountain biking (also for hiking, fishing, camping, x-country skiing, etc.) At least twice a year, I try to ride up to Franklin Bridge, it's spectacular in the fall when the larch and cottonwood trees have turned yellow, but there's so many good rides there, it's hard to pick.
I also like riding along the Kim Williams Trail.
I'm interested in road routes though - where there's not tons of traffic and the views are decent. As part of canoe shuttling, I've ridden along Big Flat Road a bit - all the way from Harper's Bridge. That's a nice ride too, but the road is narrow. (I think they're widening it though).
Does anyone know what's up with the last bit of trail that needs to be built to connect the river trail to the California Pedestrian Bridge. It's so close but not quite there. I've heard that it was supposed to be completed last fall...but it's still a leap of faith.