Unfortunately about a decade ago management began to weekly groom parts of the favorite bump run at my local ski area I skied a great deal for years. It is beneath themost popular lift at the resort. Hence the current slope is disgusting. My own preference is for really long bump slopes I can rip long lines that are modestly steep in the 40% to 55% gradient range at higher elevation with lots of skiers mining up the snow.
When it come to good bump slopes ,I have to go with higher elevation slopes with good shadowed northern exposures versus those like Outhouse that get a lot of sun or the many famous lower elevation bump runs like Gunbarrel, or anything in the East. That is because I ski bumps for the pleasure and not particularly for the challenge. And it is a fact that good packed powder bumps are way more pleasant than the hard cast in stone bumps many are only familiar with. Another issue is numbers of bump skiers. Good moguls require a lot of good bump skiers to form them. There are a great many steep slopes especially here in the West where immature steep bumps evolve that rarely receive enough traffic between usual winter storms for the bumps to shape up nicely so smooth bump skiers can just smoke through them. Otherwise just middling junky shaped snow. Additionally a bump slope needs those bump skiers to loosen up the snow. That is why the best bump skiing during any week is usually on Saturday afternoons on the most popular bump slope in a region. And that is probably the biggest misunderstanding skiers have when they talk about the best bump runs. Many tend to equate steepness and gnarliness with better bumps. Nope. Lots of bump skiers shaping and loosening up all the snow in the bumps. Snow metamorphosizes continually on all ski slopes the same way a piece of ice does in one's refrigerator. Unless the wind is blowing and moving lose windblown around, a bump slope from one afternoon to the next morning changes enough that those skiing a bump run early in the morning are not going to have quite a good experience as they might have the previous afternoon when it was freshly loose.
The above is why a bump run like Railbender at Mary Jane is so good. Good shady exposure, high elevation packed powder snow, and daily lots of bump skiers working those slopes. Of course the Winter Park area is about the closest big resort to Denver at a decent lift ticket cost, so it obviously receives higher numbers of bump skiers versus other opotentially good bump slopes in Colorado that simply don't get the traffic.
A couple comments on others choices.
Chachi639 mentioned Clipper at Sierra at Tahoe. Given the modest gradient, that is a fine slope for skiers that have never really figured out how to ski bumps out to practice on because the bumps there never get very big. Years ago I'd ski that slope until I could smoke it top to bottom then move east and start working steeper Dynamite.
dogiedoc mentioned West Bowl at Mammoth. Indeed another great modest gradient bump run and short at that which is best to practice on before going up on the Cornice or looking down Climax. The nice thing about moguls at Mammoth is the high elevation and windblown snow conditions filling in areas between bumps make for ideal bump skiing much like Winter Park.
Eagle Peak proposed ski resort:http://www.davidsenesac.com/_a-z_eva...peak_skic.html