I wouldn't say I use it the most, but I've found that despite other classes being far better-suited to direct combat, I'm an absolute wrecking ball as engie with shotgun, pistol and gunslinger, so I'll swap to that when all I care about is butchering the other team. Nobody will ever attack me first because a hardhat engie in the open is one of the least threatening things you'll see in the game... but then I throw down a mini-sentry, whip out my pistol, and erode their health to nothing in a few seconds. The only classes that have little to fear from me are the soldier and sniper because they can take out my sentries like they were never there, but everyone else (including heavies) is gonna lose a lot of blood attacking them at anything outside their primary's optimal range. If they wise up, realize I'm more dangerous than even the soldiers on my team, and start closing the distance whenever they get the chance, I'll remind them why I believe the shotgun is the single most practical weapon in the game.