I disagree with Victini being S rank. I have played across many different accounts in UU, and in almost EVERY team, I use Victini. I am saying because I want everyone to know what the hell I'm talking about. Victini is, without a doubt, A rank material. Victini has 2 primary sets: Choice Scarfed and Choice Banded. Scarfed makes it one of the best avengers in the UU metagame. There are a couple more sets, but I'll get to that after I explain these sets.
The Choice Scarfed Set: This set boasts avenging fallen friends while keeping momentum. The typical moves on this set, as you all know, are U-Turn, V-Create, Zen Headbutt, and Bolt Strike. U-Turn is great for momentum which I will not take away from Victini. However, V-Create is where the problem begins. It has a colossal 180 BP which hits as hard as Jolly Scarfed Mienshao's HJK, but the drawbacks are staggering. To lower speed and both defenses by one stage is horrid. This makes Victini EXTREMELY susceptible to pursuit trappers. Because of this, many players will be forced to U-turn, which is very easy to play around. Not only that, most UU pursuit users are indeed Dark type, which means Zen-Headbutt, another move is completely useless. Bolt-Strike, although great, doesn't hit hard enough without being Adamant or Banded. This set must be use very carefully, and even when used carefully, cannot hit hard enough due to the abundance of bulky water types in UU. Victini has competition from the other hard hitting fire types, namely Darmanitan and Arc which makes it quite natural that UU will be filled with bulk fire resists (ie: Rhy, Blastoise, Swampert). Now, that being said, of all the Fire types, I would say Victini is the best (Bar Chandy).
Choice banded set: This set hits like a semi-truck slipping on oil. It spanks pokemon harder to the point where I think those who take the hit are pornflick bitches. This wallbreaker is nothing to mess with; however, it doesn't have much utility outside of that. Snorlax walls it to hell, the premium SPECIAL wall of UU. This set NEEDS hazards support, and often times (don't quote), I feel that 2 V-Creates from a ScarfedTini will hit around the same as a single V-Create from a BandTini. This set has the same problems as the Scarfed Set; however, it doesn't have the speed to dodge attacks.
Now, there are 3 other sets I have used: SpecialTini, EBelt Mixed, and TR Sun Spammer.
SpecialTini is walled by ALL the Walls. Blue Flare, Psyshock, Glaciate, and Grass Knot. The only thing this set has going for it is surprise value. Grass Knot pretty much OHK's or 2HK's all the main counters; however, after it has taken out these counters, it is walled and easy to predict. Its base 100SpA isn't doing much for Victini. Being walled by Snorlax is not something I look for in Victini. The ScarfedTini can Flinchhax with Zen Headbutt at least!
Ebelt Mixed: Ah, not bad. V-Create, U-Turn, Psychic, and Glaciate. The utility I see for this one (in all of the games) is the break common suicide leads with a combo of Glaciate and V-Create...namely Azelf and Frosslass. It doesn't hit too hard on either side because generally you invest in one type of attack. Lowering it's respectable 100/100/100 defenses are not something you want to do. If you want a hard hitting fire type with bulk, it's Victini. Don't ruin it. If you don't mind the defense drops, use Darm!
TR Sun. Set up TR and Sun, spam V-create and Solarbeam. Its usual counters are already slower (faster in TR) by the time the first V-Create comes around--even with -Spe + 0 Speed IVs. Sun boosted V-Create does hit hard as hell, but it takes too long to set up. Mine as well use Cofag, you feel me?
-Some other problems:
-Susceptible to all Hazards.
-Pursuit Problems
-Usually has 1 hard counter depending on set
-Momentum killer if not using U-Turn (unless against Stall, but if you use stall, you shouldn't have to worry about V-Creates)
That being said, Victini is still an EXCELLENT Pokemon, and is a much better physical fire type than Darm, in my opinion. It has respectable bulk and has average utility and attacking power. I would recommend it as the general scarfer of the UU meta.
Just my thoughts, feel free to disagree.