So basically then the answer to the actual question is “Look at warcraftlogs and see what the top mage spec is higher than the others”. IMO the question “what is the best raid spec” is subjective. You can’t really answer that except for “look at warcrafts logs and see for yourself”. Arcane can be #1 for mage but what if the OP is NF Arcane? It is going to suck for them therefore making it not the best. Or let’s say Fire is the best and maybe they don’t use combustion well as a fire mage. Telling them Fire (hypothetically) it is #1 yet the main dps busrt for fire is combustion when used correctly and if they don’t then it won’t work too well for them.
We have a frost mage in my guild who competes with an amazing Boomkin. IMO all 3 specs can perform greatly.