I do have to say HOLY **** at some of the HG/SS additions. Quagsire might see more play now that it has Recover.
Mewtwo with Magic Coat seems pretty interesting.
Mew got Pain Split and Heal Bell.
Kyogre getting Block and Auqa Ring are interesting, but nothing too game-breaking.
Getting hit by a Max Spec'd Palkia Hydro Pump in Rain does sound pretty scary. It even has a chance of 2HKOing 4HP/252SpD Calm Blissey even WITHOUT rocks (48.77% - 57.52%), and a pretty good chance of 2HKOing without, but of course, there's the accuracy issue to call into question. Surf is just plain more reliable so it probably wont make too much of a splash.
Darkrai gets Headbutt. It also got NP in Platinum so it can't complain about not getting anything. Moving on.
Groudon also got Block. It got Lava Plume too, I guess he might be able to use it in the Groudon Mirror. Nothing significant.
Wobbuffet got nothing.
Dialga got Metal Burst, again, nothing too significant.
Rayquaza got Tailwind. Eh, again, nothing really signfiicant. Air Slash might have some situational uses on a Specs set, but other than that, not really.
Scizor got Bug Bite, making him have the strongest Bug attack that isn't a Heracross Megahorn (it's 60 base power exactly, so 135 after Technician and STAB), so now he officially has no more reason to use X-Scissor. Still, most Scizor use U-Turn anyway.
Deoxys got nothing significant.
Forretress got... HOLY ****: Pain Split, Gravity, Block, Bug Bite, Power Trick. Has a whole lot of new toys to play with, and is going to be even more annoying to take out, especially in Ubers where there's less of a reason to carry Magnezone.
Giratina also got some major improvements. Pain Split for healing, Gravity for... stuff. Giratina got a major buff with Shadow Sneak as a priority attack for Physical sets. Magic Coat to block a predicted status. And Destiny Bond for even more improvements.
Garchomp. NOthing significant.
Lugia. Nothing Significant. Except maybe being able to hit Forretress with WEather Ball while in the Sun or Ho-Oh with it in the Rain.
Blissey learned Heal Bell through Move Tutor. Dayum. Blissey has Wish AND Heal Bell now. Ultimate Cleric FTW, now it's more important than ever to take out that fat bitch ASAP.
Lucario. Nothing Significant.
Jirachi. Learned Gravity, which'll probably help it in OU with 100% accurate Thunders that have a 60% paralysis rate. Otherwise in Ubers, wont make much of a splash.
Ho-Oh learned Brave Bird, giving it a stronger back up STAB, but I don't think it'll be enough to push its usage up significantly.
Tyranitar learned Block, but not much of a significant add-on.
Manaphy learned Heal Bell. Eh.
I think I covered most of the significant Ubers.
My thoughts on who the most improved are: Giratina, Forretress, and Blissey, no question, with Quagsire possibly getting a chance to shine in Ubers.