Arena: Well. The map is nice in that it has something for everyone, except perhaps Sniper (although he manages to make do with what little he has). Pyro is another class that this map manages to balance out some; the availability of water makes it so being set on fire isn't so much a case of certain death like it is on other arena maps.
A/D : Steel is a contender due to its sheer uniqueness, but I'm afraid this one goes to Gorge. Dustbowl is another contender, yet every round seems to play out the same due to there being ONE totally awesome location for everything in each "area", so there's ALWAYS something in that one location. Every now and again, somebody on Dustbowl surprises me with a unique Sentry spot or whatnot, but for the most part every round feels the same. What I really like about Dustbowl is that forward base BLU gets on stage 3 right near the last point. It's not a forward spawn, but it's an area that BLU can control easily and launch coordinated attacks from, something not seen in some other maps.
Gorge's appeal is that there is slightly more variance between rounds. While it is true that there are some sections that play out the same, for the most part, there are many "Good" locations for things rather than having one "Awesome" location. This creates some variance from game to game (good considering Gorge is a one-stage map), keeping players on their toes.
CTF: Do I have to? There aren't any good CTF maps due to the nature of the gamemode, only okay ones. 2fort is okay except for the complete dominance of Sentries over the Intel (if the sightline was just a LITTLE bit larger to allow for picking a Sentry off at range, it would be okay), but personally I prefer Turbine. Turbine's issue is that it suffers from tunnel syndrome; it's simply too enclosed and doesn't offer enough vertical movement. If it were more open, it would be a lot better, although unless the Intelligence was also open, the tunnel syndrome would just move there. However, if it were open, it would lose the ability for surprise ninja Scouts that can drop from the ceiling, grab the intel, and run, so to fix that would be to break it in another way.
CP: Coldfront. It's an odd choice (Badlands comes close), but the only thing I don't like about it is how fast the last is to cap. If it were a second or two longer, it would be bearable, but essentially nobody has a chance to really defend it. It doesn't create a lot of white-knuckles moments where you just BARELY stop a cap; once somebody has touched the cap, it's game over. Otherwise, though, the map is solid, has lots of flanks and there are opportunities for every class to function.
Freight is another contender for this crown and actually rectifies the issue I have with Coldfront. However, even though the last points are great and awesome, Mid just... isn't fun. Mid doesn't seem to have a dynamic combat element about it. Every class functions the same at mid, and the water isn't really... useful. If Freight had arena_well instead of its current mid, it would easily take the top spot. But the mid is such a central area for "fun", and for mid to be the least fun on the entire map isn't good, since most of the fighting will be for mid. Coldfront's indefensible final cap is only a minor fighting area that, while important, isn't fought for NEARLY as much as mid. While I understand the reasoning behind the quickness of the cap times and the fact that Badlands is the same, I don't agree with it and think that turtling can be dealt with in other ways, or that the final CP can be handled like an A/D map, which Freight does nicely, IMO.
KOTH: Nucleus. The reasons for this are many and said already.
PL: Like so many, Badwater. Badwater has a great opening in that it gives RED a lot of high ground at the very beginning, but are forced to rely upon and defend teleporters if they want to keep it due to the distance from their spawn. Badwater is also great because of the lulls between frenzied bouts of combat. Many maps make the cart the center of a never-ending spam brigade, while Badwater encourages people to control the map, area by area. While many people don't like this whole "Control an area and then move the cart up" strategy, it works for this map and encourages most of the map to be combat-ready, even if most of the focus is on the actual points themselves. That's the reason why I prefer PL to A/D; A/D has a very specific focus on a very narrow area and creates a lot of "dead space" between points (The maps that I don't see this on are Gravel Pit and Gorge, Gorge I mentioned, but I didn't choose Gravel Pit because of how badly it splits up the teams), while in PL, every bit of the map will have SOME form of combat going on around it at one point or another, even if it's just a minor skirmish (like on Badwater's tunnel).
PLR: Pipeline. I haven't played much Nightfall (if at all), and I can't stand Hightower. Nothing really unique about Pipeline that stands out, other than its theme. The only thing I don't really like about Pipeline is stage 3, which I can't stand, much like Hightower. Being as Hightower and Pipeline stage 3 are somewhat similar, I think there is a lesson there on what not to do.
I think that's every official gamemode except TC (which wasn't chosen due to there only being one TC map), but correct me if I'm wrong.