Black Creek is the hardest to get to, its all bushwhacking. There is a private lake at the bottom of it, and I don't believe their are any real trails along the creek. Brush Creek is probably the best access, you can then bushwhack to Black Creek avoiding the private land. I agree that Piney over Kneeknocker is the best way to get high in that drainage and access the peaks, its how I plan to get to Bubble Lakes whenever I finally get back there.
Brush Creek road is not too bad except for the bottom if its muddy. The first 1/2-3/4 mile is probably the worst. Right now the upper half the road is covered in about 100 deadfall trees (no joke, its a mess!!) and the gate is still locked. Maybe by later this summer they will clear it out. I was just at Lost Lake this weekend (for Peak N, awesome peak!).
Brush Creek is also probably the best way to get to Slate Lake, but its long. Boulder and Rock Creek are the alternates, but have more vertical gain and I don't think you can park at either one, you have to park at CO 9 and walk up the road. I think you might be able to drive up and stash a pack, then drive back down - or bike the road, I think some people do that. Slate is awesome and Peak L one of my favorites in the whole state.
The east side access points are hard because of all the darn private land along CO 9 north of Silverthorne. I have actually climbed many of the "east side" peaks from Vail including Keller (from Bighorn), Peaks X and Z (from Pitkin), Peak P (from Booth) and Eagles Nest (from Piney). I even tried to get to Q from Booth but the ledge is pretty sketchy going that way. When I go back for Peak S I will come from Pitkin, and for Bubble Lakes area peaks, I will come from Piney. Of course I live on the Vail side so part of that reasoning is to save on driving - Peak N was only 17 miles "line of sight" from my house by I had to drive almost an hour to get to the trailhead, then hike in 8 miles to its summit!