I just read through this entire thread, and despite the main meta being OU, the post is just titled "Stall in BW2" so I figured I'd give my two cents, cause I've tried stalling in every main tier.
-In OU, I just generally can't find/haven't come across a stall team yet that would give me a better advantage to win than a balanced/bulky offence team would. I've tried rain stall, sand stall, weatherless, and after reading this thread, sun stall :P To me, sun seemed the most anti-meta, and possibly the most effective if played correctly - but it could be said that any stall team would be effective if played correctly.
-NU is where I probably have the most attempts at stall, and if the sheer number of potential threats in OU is worrysome (which it is) NU is truly a nightmare. There are a number of pokemon that just break right through stall regardless (bulkup, sub, roost braviary - which I have run before on stall - literally laughs at everything if it's saved for last) and even offensive based things like banded emboar, scarf sawk, sub golurk, lifeorb cinccino, specs emolga, etc, etc... So many threats. The hardest thing to overcome about NU stall, is that there are just SO MANY POKEMON. It is literally impossible to counter even the main threats with your team of 6, let alone the other 200+ pokemon in NU.
RU - bleh, I really haven't played much of RU lately. I used to at least try, but ever since nidoqueen decided to carry life orbs and run max special attack, mixed in with hitmonlee and sceptile, I just gave up :/
UU is a different story though. Some defensive powerhouses that just wouldn't work in today's OU meta, have fallen down to a place where the offence doesn't hit as hard and as fast as standard. There are still threatening attackers in UU, but generally you CAN counter pretty much any relevant threat in a team of 6, fully defensive, shuffle-stalling pokemon. Here's a team I've had amazing success with - feel free to critique it:
http://pastebin.com/PRs9uiUS
Plus a few matches:
One that went well -
http://pokemonshowdown.com/replay/uususpecttest4736186
One that required improvisation -
http://pokemonshowdown.com/replay/uususpecttest4737286
I feel stall works best as an option in UU, soley due to the lack of overwhelming power, and number of offensive pokemon. Just my opinion, feel free to yell at me for anything I did wrong, I wrote this late an tired :P