Are you able to log every day and make Xmute? Are you able to gather all mats for truegold? Do you have miner/JC for red and meta gems transumtes to be worth? Are you in a rading guild? Do you or your guild prepot?
So if you log every day and can gather/buy mats for truegold then go Xmute. If you can get your hands on common red gems and heartblossom make rare red gems and meta gems(depends on server) so in this way Xmute would be most money awarding.
On the other hand potion masters are rare(at least on my realm) so if there is potion market maybe potion mastery would be better.
Flask mastery is the worst of 3 imo. Most of the guilds are using caludrons by now so you are not likey to sell a lot of flasks. Plus guilds made a lot of flasks to unlock caludrons so they might dump them on AH if they are not planing to use em. So Flask mastery is not so good imo atm.
I have all the specs since I made 3 alchemists just for that purpose.
Transmute is good money
Elixir and Potion are good for raiding (if you have access to guild cauldrons then I would say potion even better since flasks from cauldrons are really cheap)
Potion imo. Pots are selling at 15-30g each and thats with 10g+ profit per pot. Then there's Potion of illusion, more then doubling it's cost and selling 500+ per day.
I wouldn't go with flask, simply because the majority of people who raid can get that from guilds using cauldrons or they are purely sponsored by guilds..
Xmute if you got time to do it everyday, else potion mastery.
I have elixir specialization and it is utterly useless. Not only do Flasks now sell for very little compared to the raw material costs, and most Guilds just use cauldrons anyway, I barely get procs. I made about 88 flasks out of material for 80. Not sure if it's just me being very unlucky or elixir specialization being even more useless than expected.
Only reason to be elixir master is to proc flask cauldrons for raiding (imo)
I make a nice profit being xmute spec but i went xmute spec to log in once a day make truegold then log out.
Potion would be best bet if they sell well on your server.