Check this
wiki page. We're all basically just cribbing from it anyway. Lately I've been using the DFhack command "autodump destroy" incredibly liberally.
I'll add my biggest considerations, though, in roughly their order of importance:
embark size - I think this trumps just about everything else. More space means more of
everything else. I do mostly 1x3s these days.
population - more dwarves with more crap doing more things. It hurts! My pop cap is never above 140 (just enough to become a mountain home). Sometimes I'll tweak "strict pop cap" depending on my ratio of dwarves:babies:outsiders. If you don't care about becoming the capital, I say go much lower. IMO there isn't a lot 140 dwarves can do that 50 can't.
activity on surface - a glacier will be better than a dense forest. I imagine this is less important the smaller your embark, but still relevant.
flowing water on surface - as much as I love a natural waterfall or a lovely brook, I've decided it's not worth the cost.
fort design/pathfinding - use traffic designations and keep the forts relatively simple
If you wanna cheat (just a little) or alter the game:
temp/weather - turning them off can be huge. A glacier with no weather or temp calcs is the best possible fps embark I've found. Obviously it makes the glacier easier to not have those but I've already proven to myself I can survive a vanilla glacier
items - destroy everything you don't need! This can be done without commands (just throw it into a volcano!) but I can't say enough good things about DFhack's "autodump destroy" command. "clean all" also helps and removes gross vomit as a bonus.