Comp Tutoring cause I've been there since I started anything Smogon related and watched the community grow and made tons of friends. Challenges for the same reason although it's a more laid back environment and easy to make friends cause generally everyone is pretty open to individuals. I have another favorite but it's a private room. I hang out in OU as well but it's really hectic sometimes and makes my brain hurt from reading the logic that people use on matters. Regardless though some players I respect and are friendly with reside there so they make life a bit easier and keeps me going.
Monotype is my haunt in showdown, everyone there is friendly and funny and are willing to help out with movesets, nature's and general team building and mostly it's a laugh unless boTTT is on duty lol :)
Breeding room is probably my favorite room because the regs and higher-ups are fantastic users. It's a chill place to hang out and talk about what you're breeding and/or how much you're sick of seeing [insert pokemon here]'s face. Also, it's (probably) the only room in which a Luvdisc could actually make an attempt to win something (so close, but yet so far!).
"[12/18/2014@17:32:24] Hero_of_Ocs: this room is 20% o/ 50% messing with aspire 10% complaining about wifi 10% sring 5% redirecting people to wifi and 5 % actual breading" (credits go to hero and ih8ih8sn0w for the quote :D)
Lobby iirc tbh imo. Something about ebola, smash codes, shrek, pre-pubescent swearing, ban after ban after ban, random porn links, religious fanatics against ~insert blasphemy here~, marc, and the people who try to talk about Pokemon amongst the chaos really gets me going.
In all honestly, I have a general love towards the Showdown! rooms as a whole.
I'll just mention the one's I spend a lot of time in lately: Old Gens, for Gen 3 battles, Monotype, because I feel strong with Fighting (heck, one of my 4th gen OU teams is monotype!) In-game, because I play Fire Red and Sapphire.
Eheh, I don't want this post to be too long, but there are a couple others. You get the idea ~.==.O !