It's hard to pick a specific vehicle, but it is undoubtedly from Mechwarrior.
In a general way, I'd say my brakedown goes something like this:
Timber Wolf
Cauldron Bourn
Dragon
Highlander
Wolfhound
That's in a general order of how often I used them. The Timber Wolf wins primarily because it was my favored mech before the expansions in MW4. The Dragon and Highlander would be higher in the list I suspect had they not been a part of the last series of mechs officially released for the game.
Plus, my time behind the controlls was dictated almost entirely by the map and scenario being played. The Timer Wolf was a no frills, bog standard mech (though I did vary from the popular trend by losing a pair of ER Large Lasers in favor of a pair of LRM-15's, which allowed my mech to maintain a faster movement rate and suffer fewer heat concerns).
The Dragon was my mech of choice for any of the close quarter's non-urban maps because it's reliance on projectile weapons meant it was a perfect high intensity mid ranged mech. That it was fast and durable was a plus, and the RAC5/UAC2 combo was surprisingly effective at shredding armor.
The Highlander was my mech of choice for Urban combat, and my usual mix of weapons left many perplexed. It had a fairly low alpha strike potential (as did really any of my mechs) but it made up for it simply by being able to start a fight at 600m (as long a shot as you'll get in a city) and maxing it's potential damage before getting inside anti-mech shotgun range (which most other players were using). Varied weapon systems with fairly high rates of fire also helped keep the enemy's aim off balance since I could hammer them with one of the groups every second or so.
The Cauldron Born was my later mech of choice for long ranged (especially in the warmer maps) fights. While it had a pitful alpha strike, it's combination of weapons (2 LRM-15, 2 ER Large lasers and 2 UAC/2's) meant that I could wear away armor with alarming effeciency, and the ultra high rate of fire ensured that once my guns opened up, the other player would have a hell of a time doing anything about it. Plus, most of my arsenel was effective beyond ER Large Laser range - an essential tool of survival.
The Wolfhound was my favorite mech for use in Clan vs. IS games. As a light mech it rarely did any good in normal games where most everybody would choose heavy or assault mechs, but in league play where there were team tonnage limits it proved to be immensely effective at high mobility harassment. A few shots from the PPC would generally draw all the attention in teh world, and the high speed of the mech ensured I could keep one step ahead of my foes for qutie some time, allowign my team mates plenty of opportunities to wear them away from teh flanks.