I voted "more than 20.5 (Please explain)."
One of my favorite playing cues is a 21-oz Fury NR-08 with a Tiger stack leather wrap. But because this cue is rearward-balanced (I can balance the cue on my finger placed right at where the forearm ends and the wrap begins), the weight is not as noticeable.
I used to play with 21-oz cues anyway in the mid-1980s, when in a lot of the recreation centers on the US Navy bases (I'm an ex-sailor), the house cues tended to be on the heavy side.
Interesting that in this poll, the OP seems to think 20.5 is the upper limit and anything heavier than that "needs explanation." 21-oz weighted cues are actually quite common, and, when ordering a cue, is one of the common weights that are listed as an option.
For me personally, anything lighter than 19-oz makes my stroke feel like it's on roller skates -- much too light. A heavier cue deadens that zinginess, and actually gives my stroke more feel; I can better gauge the hit.
I've read somewhere that Shane uses a 22-oz cue.
-Sean