Most of the requests for my fandoms do not currently have letters, but out of what I can see:

Best: Either that letter from fandom A that gives me free reign to write the weird AU of my heart or that letter from five minute fandom B that loves all the same things about the premise I do and has fun prompts.

Worst: That letter from ongoing fandom C that is only interested in a very specific aspect that is almost certainly going to be covered by canon, possibly in the very near future.