A "catch all" sort of temperature would be 70F/21C or a bit less
If you google for the lalvin yeast list, you'd see that most, if not all, are fine there, but it also helps for things like the anecdotal evidence that popular yeast like D47 have been found to produce fusels above that.
So as jackfrost says, it's yeast dependant - mainly.
It's also worth remembering, that further to that, that if you pick a yeast that's more commonly used for red wines, you can get good meads but they often like it a bit warmer, as do some of the more specialist ones that will produce some of the higher levels of alcohol.
Most of the more experienced mead makers, especially those located where it tends to be warmer, find they get better results to keep the temps at lower levels. As it seems that the yeast do a better job and produce smoother brews.
I don't generally have to bother as the UK's temperate levels mean that temp control aren't usually necessary.
So you pick your yeast and take your choice, even if you pick a good yeast for traditionals like K1-V1116 which has a very wide temp range.
Also its worth baring in mind, that often, the higher the temp and the stronger you make a batch, the longer it can take to age a mead......
Just my tuppence worth......