Alright, so after about a week of posting fluffy content let's see if we have enough people to scrape together a proper discussion thread.
My favorite gamemode is 5CP - I dearly love Payload and KOTH as well, but I like 5CP the best because of the back-and-forth it generates; you will be able to participate in both attack and defense at the same time. KOTH also has this, but it's faster-paced and easier for one team to camp the point, in my opinion. I actually prefer longer matches as I think it allows for more interesting strategies and counterplay to come about as opposed to matches where it's a blitz, at least in pubs.
My favorite map from this mode is Process. It has adequate flanks, different environments for different classes to shine (outdoor, indoor, and hybrid spaces, as well as lots of changes in elevation), and is relatively "streamlined" despite all that. It fosters a decently competitive environment even in an unorganized public server.
(Runner up goes to Powerhouse (3CP technically, but it's my post so I can break the rules if I want) for mindless pub play, I've had some legendary matches on there.)