my gf has a m/s/p and I have a m/w/p both well geared, so I have a lot of time on those. Just recently we leveled a m/w/wd as well, so I got to try out the m/wd elite which blew my mind on a good connection/computer. So here is my breakdown
m/wd- Ridiculous with two burns, and the 5 seconds of glory (which I like the name and am stealing it :p) has a cool down of 3 min, so you can 3 min burn, full burn, then 3 min burn in a longer boss fight... Plus another instant and a buff that does increase stam/int. Probably best DPS, and my second favorite
m/w- Buff to magic dmg, buff to crit dmg, buff to crit rate, reduce agro skill, buff to magical attack.... Lots of buffs to make your dmg through the roof on a burn. Now after the burn you are kinda tapped out, however there is the 50/50 elite that every 30 seconds, if you are spamming flame you get a 40% faster cast for I think it is 10-15 seconds? giving you mini burns often to keep up the DPS. Would it outdps the 5 seconds of glory over all, maybe not, but for me I don't like waiting for doing decent dps :p Which I say decent cause mages need some love.
m/p- This used to by my favorite and I still use it for some applications. 5 second immune may help in some boss fights (only one I can think off the top of my head is 4th in CoB instead of having to pop seren), healing yourself is pretty pointless in instances. Double hit on flames more, buff magic attack and short buff to int using magic drain. Good for a burn but then fizzles out. I still like the combo for hunting/questing, and of course siege. Which is where I think it shines the most.... mostly due to the immune, instants, and healing.
m/s- buff to crit rate, extra slot to stat, lots of spells, the "bubba" as we call him for a nice boost in speed, and after extra dmg. That works great and could be used to have a second "burn" phase, like the m/wd. Can use the bubba, with minimal buffs, to do decent dmg, then when speed boost is gone, buff to do full burn, and kick It when dmg buff sets in (since you really don't need that speed up to hit .5 second flames, instead use it for the dmg buff after getting a mini burn). Then yes the extra dmg to make it better sustained. Also with a bow, you can put in an extra crit rune, dont know if recon works as well, which would help aggro in burn phase. I would also put it at number 3 for dps, and for instances my personal #3, but that is just cause of my playstyle..
All 3, m/w, m/wd, and m/s, are good for endgame instances, and while one may shine a little more here and another there... I think all 3 are good combos
Edit to add: this is all to set to analyze beyond the normal mage burn which all mages get. If you are just burning then most of them are the same with the exception of m/p who with double crits will jump back to near the top of the list. I just figured an overall analysis and long term fights. Cause lets face it, if you group is successfully burning bosses quickly, your worry wont be on long fights. Short fights it is pretty fairly balanced for mages, and the difference in damage wont make or break you... Unless you are just competitive