For me its none other then Oubliette. Glitchy as hell and most of the times im stuck fighting on that map,its a race to who gonna get the Omega Cannon first.
either that (mostly cause of the glitchers) or combat hall. god combat hall is annoying.
Combat hall is only good for 1 on 1 fights and only if you play in Friends and Rivals and also if you know that the one you face wont use the wall glitch.
Combat Hall really isn't all that bad, it's just that in wi-fi battle everybody uses it for cheap kill whoring. The Alinos Gateway is annoying because of the cheap snipe-spamming that plagues it... but it's actually a pretty cool arena.
I don't like the small Arenas like Incubation Vault or Sanctorious because there's no room for strategy- I like being able to retreat far away from the battle when I'm low on energy, in order to minimize deaths. Places like Compression Chamber are confusing and tend to be plagued by alt whores.
Obliette is annoying because there's no fun fighting with people who just run to the top to get the Omega Cannon- I did like the area below the Cannon, though, because there's a lot of places to duck in cover.
My ALL time favorite Arena, however, is Harvester. It's one of the few Arenas that were created soley for multiplayer, which earns it a few points alone, but beyond that the place is wicked cool. It has the Arcterra music, which is the coolest, and lots of nooks and crannies to explore. The central area is great for close-quarters combat where the areas on the outside of the structure is very open and good for sniping (sniping is acceptable if you don't whore it.) The architecture is really good for eye-candy, too.
I also like the Data Shrine, Fuel Stack, Elder Passage, Alinos Perch (basically a smaller version of Elder Passage), and a few others I can't remember.
I rarely use wi-fi; bot battles are by far more fun.
Combat Hall is my worst map. DAMN GLITCHERS AND HACKERS! I once got Omega'd there. And Oubliette. As a frequent/mandatory Noxus user, I seriously struggle getting to the top. Other than that, I haven't played in many other (ACTUALLY GOOD) levels. Processer Core, however... It's like it was made for me.
My worst map is probably combat hall. All the glitches really ruin it and anyone using samus spams the crap out of you. Plus, theres WAY too much health so noobs can just run around and get health until the match ends.