If it was Team Fortress of the Quake era, easily the Pyro. While the class function remains intact (which is skirmisher - a class that operates in a shallow defensive posture or shallow offensive posture to hamper incoming assaults or disrupt defenses), the change in how this was accomplished was far too severe for my liking. Previously, the Pyro operated in much the same way as a soldier and most relied on the incendiary rocket launcher to deliver damage (which were quite week. In Team Fortress Mega it took 3 hits from this weapon (allowing time for the resulting DOT to work) to kill a sniper. Thankfully, in Mega this damage had no regard for obstacles (meaning you could apply the DOT through cover if the target was close enough). While the class was at it's best on the defensive side of the skirmish line (it was hard to get past a Pyro with all your health. The only classes that could beat it in a straight slugging match were the soldier (far more firepower) and the Heavy (far more health)), the combination of lava traps, napalm grenades and the rest meant you could run rampant in an enemy base dealing significant (in terms of HP lost) damage throwing a full defensive effort into disarray singlehandedly.
My second choice would have been the spy, but the same can be said. While the purpose of the spy is identical, the lack of utility items severely limits your options that lead, quite necessarily, to opportunistic HVT removal or a straight combat class using the disguises and whatnot to help make up for the lack of firepower by ensuring the first shot. Furthermore, the general game world has changed such that surviving while disguised while behind the lines for any length of time is difficult at best. Basically, the bottom line is I miss my ammo trap (you stole all the ammunition from an enemy save for the round currently in their weapon, looked exactly like the backpack of ammunition dropped when an enemy died except it did not rotate), the grappling hook (a one shot ticket into or out of a sticky situation), the feigned death (more than once I got an absurd number of kills on well by feigning death as a sniper) and my hallucinogenic grenades (they didn't do much damage but you could easily fill much of a smaller base with gas). The one gadget that was kept was the regular grenade and it was simply replaced with the sap as your average spy saved their grenades to kill sentries (throwing a grenade was the only thing you could do that wouldn't break your disguise).
As such, it would be very hard to pick. The changes were probably for the best in terms of balance (though I assure you one of the funniest sights you'd see in video games was when a spy grappled onto one of the legendary dogs that kept people from camping outside the enemy fort in 2fort5r (the most popular iteration of the map that lives on in the form of 2fort) who then feigned their death. The dog, wanting to attack the spy (they aren't fooled by disguise or feigned death) would constantly try and turn around, but since the spy was attacked to the dogs rear it couldn't ever reach it. Thus you had a dog running madly around in circles with a person being dragged behind.), but they also excised much of what I really liked about the game. The heart of it all is still there but having been so highly refined it makes it hard to pick as each class is a purpose built weapon these days.
I suppose, where I forced to choose, I would probably opt for the soldier. I'd like to pretend I'd choose something else but the options aren't appealing. The Heavy is amusing for a time but the pace of play is too deliberate and plodding to occupy me for long. In exchange for useful combat healing (something the medic lacked when you had to beat your friend with a medkit or, alternately, with a magic axe of healing) you have poor combat utility thus making you little more than a mobile health dispenser. The engineer involves too much sitting around and waiting for things to happen, something that exacerbates the paranoia inherent in the job that leads quite naturally to personal insanity. The demoman has always been a defensive class at heart and I've never cared to master the sticky to the degree to make it a viable offensive class. The scout just plain annoys me either way. It's fast and twitchy and utterly uninteresting. As for sniping, I'll leave that to people more patient than me.