I personally liked MvM as a cooperative gamemode which added more depth to the classes in the form of upgrades. Easily accumulated a couple of thousand hours just playing that either on boot camp valve servers or community ones with custom maps and plugins (off the top of my head UberUpgrades and Potato come to mind).

Didn't particularly like mann up as a) it costs money for each tour and b) it heavily incentivized you to win by giving you the chance to win a expensive shiny golden gun, which in return made players just develop some meta which they religiously sticked to and vote kick you for deviating from it even a tiny bit, or god forbid making even the slightest mistake. From my experience most players were in it for those items which they could sell for a profit, actual fun be damned.