http://www.wowwiki.com/Instance

has all the instances and their levels.

I do DM up to 15. then stockades 15-25. this is great as it has no run-to time and shops a plenty to get rid of all that lovely loot I then did SM from 25-40 (you can only use the meeting stone after lvl 30 so this is a drag as its so far from anything). From 40 I'm in ZF (I never do uldaman or mauradon but thats my preference) At about 45 I'll go to sunken temple then at maybe 50 scholo and brd will be my homes (i dont like strat)

you will get lots of answers from lots of people I think but thats my plan.

Mages are brilliant all the way up to the end of SM as they aoe everything. ZF it starts to get a little tougher for them but of course they can still do well