For uniqueness, Masterminds; there are pet classes in other MMOs, but the flexibility and variety in Masterminds was head and shoulders above other games.
For the other ATs, specific characters and powersets stand out. My AR/EM Blaster, who, with my Psi/EM Blaster, were built out to see if I could make Jack Emmert's assertion "range is a Blaster's defense" true. To a fair extent, it worked, although the Malta Group, whose Gunslinger lieutenants and bosses could, with pistols, outrange a Snipe with Range Boost added on, were always a pain. And it was always entertaining, on a Rikti mothership raid, to stand in the middle of the bowl on the ship, hit Range Boost, and Full Auto the Rikti standing up on the lip of the bowl. The Psi/EM Blaster became my favorite for ITFs, because all of her attacks fell neatly into the Rage hole, so when the Minotaurs and Cyclopes popped Unstoppable and everyone else's damage dropped into the low two digits, I was still doing three- and four-hundred point shots to them.
My DM/EA Brute I wish I had more time to tweak her build on; I feel I could have increased her survivability and made her a more rugged fighter. I was able to tweak my TW/EA Tanker, capping resistance to Energy and Negative, which made any mission against Rikti almost a cakewalk. With her I had to be careful not to fall into the Tanker equivalent of scrapperlock. Although the slow animations meant that, unless facing opponents with lots of HP or defenses, they would all be on the ground before she could get more than an attack or two off.
I could go on about my Dominators and Scrappers and the like, but in the end I have to say that there's no one AT that I'm really going to miss much more than any other; it all pales beside the loss of the game as a whole. Although if I were to do it again, I don't think I would have the dogged determination to get a TA/Arch Defender to 50 and an Arch/TA Corruptor to 29... while the combination of Trick Arrow and Archery is a ton of fun in a group -- you don't take down a lot yourself, but it's very easy to see how much faster things go down around you with all your debuffs flying around -- they require a very careful, measured play style to survive well solo at the higher levels.