what is the best secondary class for a monk?

This has to have been asked before, you might try the search, but...

Lots of monks don't use their secondary at all. If you do use your secondary, it should probably be only one skill for energy management. Mesmer and necro are good.

I wouldn't recommend warrior at all.

mesmer for inspiration skills.

necro for err...the less frequently used offering of blood.

I started out as Mo/R, switched to Mesmer after ascension. Never used my secondary till then, and now I use my secondary quite a bit = Mantra of Recall {E} and Inspired Hex mostly.