It really comes down to personal preference
The top 3 would be
Blood Elf, Troll, and Orc.
The Panda racial CC is nice, but keep in mind that it shares DR (Diminishing returns) with Polymorph.
Blood elf- Highest base intellect, more SP is very helpful. Arcane torrent (2sec aoe silence) is decent, but only good on pushes; useless when casting from max range. great in arenas cause of arcane torrent
Troll-Low base intellect. Notable benefit is u get pretty much a second (not nearly as good) lifeblood; Berserking increases your spell casting by 20% for 10 sec, can be used with Lifeblood and will make ur arcane blasts go from 1.9sec to 1.2 sec. this would be the best if berserking had the same CD as lifeblood. unfortunately berserking has 3 min CD whereas Lifeblood has 2min cd which makes it hard to use at the same time after the first time u use them
Orc-low base intellect. gives blood fury, which increases ur SP by 21 for 15 seconds, and has a 2 min CD unlike troll racial.. u can basically use this whenever u use lifeblood to put out maximum dps, can even macro it to ur lifeblood
others:
goblin has high base intellect (same as belf) but i dont find the goblin jump that useful as a mage, u have blink and blazing speed if u choose this talent.
undead- the racial is nice; basically a trinket on 2 min cd that only works on fear/charms. i dont find it as good as the others tho especially with ho wmany dots there are that can break the fear ur in. it has touch of the grave, a pretty small dps increase but i have heard of it breaking ur frost novas; it procs on damage so ur frost nova does damage and procs touch of the grave which then breaks ur frost nova...,. blizzard really needs to fix this
basically all of them have their benefits, jsut comes down to what u prefer
