Hmmm, Fire>Acane>Frost 6.1.
I always loved the fire's gameplay. Arcane 6.1 was so enjoyable and Frost was way better than MoP.
Now, Fire>Frost>Arcane. Not because I love 2 button weaving as frost, but I can't stand PoF/AM/t18 rng, especially the first one. Fire has come to a point that has a much more stable opener than Arcane with a guaranteed crit buff up for PC. That doesn't mean it overpowers Arcane, but I really despise PoF. It can skyrocket you or disappoint you immensely.
In general Fire, but really it ebbs and flows with all 3 depending on what has been "best" for a while since then I get bored of that one and want to play something else.
The only spec I enjoy for Mage is frost. I mained mage from Ulduar to ToT and played whatever spec was best, but around 5.2 I finally reached the point where it's just not worth it to me to play anything other than frost now.
All three can be good or bad depended on case. e.g. Frost can be boring but also extremely consistent, it is not going to surprise you and you can't easily fail to rotate with it. Arcane can fail to rotate properly but it's very fun when it works well. Fire can devastate on AOE but it can be bad for bursting on single target.