I'd almost kill for a U binned 3W. I haven't seen one myself yet.
As for the most common:
In the 1W's, the R3H is probably to the most desirable, if you were going to choose strictly according to bin. In real life, I've found the Q3J's to be the most consistent in color of light and brightness. If you find an S, you're lucky. I've never seen one of those either. Keep in mind though, I haven't been around that long.
In the 3W's, the TWOJ has probably been the most sought after. If you find a place to get 'em, let me know!
In the 5W's, the X3T is it.