Voted "Other" because I've tried 'em all and don't always use the same ones, sometimes use none at all.
Each has its advantages/disadvantages.
WMP 10 isn't as bad as it's reputation, but not great either. Tried WMP 11 but rolled back to 10 because 11 was a bit slower and didn't really add anything even though it's bigger and more bloated. Both versions have a sometimes-problem of randomly refusing to transfer a song or two, insisting the file is "protected" and WMP "cannot verify the license". Well duh, maybe it can't "verify the license" because all my digital music is ripped from CDs with EAC and none of it is "protected". I don't use subscription services and don't buy low bit rate DRM-infected crap. WMP (both 10 and 11) do this randomly, one day a song will transfer just fine but next transfer the very same file will be refused, and it's never the same file. Doesn't happen very often though.
Tried Media Monkey and it's OK. But honestly it looks and behaves so very much like WMP that it seems people use it just to avoid WMP.
It does not have the problem of sometimes refusing to transfer a file, but it does have the problem of sometimes mysteriously creating duplicates for no particular reason. Just like WMP, it doesn't do it all the time, just sometimes. Intermittent problems are the toughest to figure out.
WinAmp used to be good but the newest version is big and bloated, though not as much as WMP. Lots of really great skins but the novelty of that wears thin pretty quickly. Unlike WMP and Media Monkey it transfers fine, but sometimes WinAmp crashes for no particular reason and again it's nothing repeatable but it happens on two different computers with two different configurations.
I like iriver Plus3. Much smaller and simpler than the others, it never refuses to transfer and never crashes. Designed for UMS devices, so it won't work with any that are in MTP mode. I also like Windows Explorer, and with my UMS players usually just drag and drop folders, then open up iriver Plus3 to create a few playlists. It can be used that way, just to quickly create playlists on an almost-full player rather than to transfer files. Using Explorer alone is fine for those who either have small collections or very organized large collections.