My mage has the most transmog sets of all my characters, and she doesn’t have that many. She has lots of pieces that look great, but not enough to form complete sets. These are my favourite sets so far.
The first one is the priest’s tier 10 heroic version. As I am a mage, I have to use the look alike items that drop from ICC 25H. As you can see, I have no shoulders that fit with this set, as there are no look alike for these shoulders. Someday I will find something to complete this look.
The second set is the mage’s tier 7.5 that drops from Naxx 25 and Obsidium Sanctum 25. The belt isn’t part of the original set and can be farmed or bought on the Auction House.
The third set is the mage’s Relentless Gladiator PvP gear from WotLk. You can buy it from the JP vendor in Dalaran. The belt I use drops off Kael’thas in Magister’s terrace. There’s also another belt that looks fairly good with this set and can be farmed or bought off the Auction House.
The fourth set is the priest’s tier 10. Yet again, this is the look alike items, which can be bought off the JP vendor in Dalaran as well. The shoulders aren’t part of the original set, but the Warlock’s tier 9 shoulders look alike. They can also be bought for JP in Dal. (However, if you go Alliance, those shoulders change and no longer fit with this set.)
The fifth set comes (in part) from Black Temple. The helm and robes are spirit cloth that drop in the raid. I found the shoulders, gloves and belt on the Auction House. The gloves are black and small, and can be used with other sets that require discreet gloves.
The last set consists of the mage’s tier 5, except for the gloves and bracers, which I bought on the Auction House.
There’s a lot more set I want to get, like the mage’s tier 4, which looks like this:
The chest’s token drops off Magtheridon’s Lair, the shoulders drops from Gruul’s lair, and the gloves can be found in Karazhan. (I haven't figured out which belt I will be using with this yet, so in this screenshot, Jori is wearing the one that drops from Tempest Keep and matches the mage's tier 5.) Happy transmogging!