Kebler is the largest area for sledding, and is where I typically rode when I lived there. Lot's of different types of terrain out that way, so you can keep it more mellow & ride the various meadows, go high mark Mt Owen in the spring (note: I've never seen bigger avalanche debris piles), or tree ride all over the west side of Mt. Axtel (or even Carbon). It's a haul, but out by East Beckwith is awesome too. To get to Kebler's trailhead, drive into CB, turn left on Whiterock Ave, drive through town, over the bridge, and continue on until you reach the trailhead.
Washington Gulch is an option, but there are a lot of "off limits" areas out there and you're supposed to have a separate sticker (just go to the FS office in Gunnison and ask for a Washington Gulch sticker, they're free, they just want to explain the closures). The riding here is fun but limited, and a lot of the fun stuff is below the ominous faces of Gothic Mtn. But ride up to the south ridge of Mt. Baldy gives you an awesome overlook of the valley (call it top of the world).
Going up Slate River Road to the trailhead there and you'll see a lot of xc skiers (actually same with the Gulch), and I don't usually head up that way until spring just because the terrain is so high consequence, but it is awesome.
I never rode any of the areas you listed. Lake City is a town, on Highway 149, in the northern San Juan Mtns. The sledding out of there is probably awesome. Sargents Mesa is south of Highway 50, east of Gunnison, up on the continental divide. Spring Creek: go to Almont, turn right, drive up to Hormel's Resort, spring creek joins Taylor River there. I imagine the sledding is up the dirt road on the left, there's some awesome mtn biking up there (Doctor's Park).
Hopefully that helps some.
But to keep it simple, I'd go up to Kebler and try to meet up with some other riders.
Also, check this site before going out every day:
http://www.cbavalanchecenter.org/
The terrain out there is no joke and they get a helluva lot more snow than the front range.
