Although I have little experiance within UU, I have quite a bit within NU, and used to use a Sunny Day team before the drop of Charizard and Magmortar.
Victreebel is an absolute monster, both in, and out, of the sun, and, most importantly, is the only reliable physical Chlorophill abuser.
Due to Victreebel's ungodly speed in the sun, anything that dislikes Leaf Blade will have to switch out, usually, allowing for a free Swords Dance. While setting up in the Sun may seem against the premise of the idea of abusing Sun, it's vital to abuse OUTSIDE the sun as well.
The key to that is Sucker Punch. Victreebel is one of a very small group of pokemon who can abuse Sucker Punch + Swords Dance. While Absol gets STAB on Sucker Punch, Victreebel has higher defences, and can abuse the sun. In addittion, this makes Victreebel less fearful of priority [Although it resists Mach Punch, Vaccum Wave and Aqua Jet]. Outside of the Sun, Sucker Punch can be used to clean a weakened team.
Victreebel's final slot is somewhat dependant on the team. Weather Ball is better for while the Sun is up, but HP Fire is better for while it's down.
Sludge Bomb is worth a mention due to being able to OHKO Leafeon if you are weilding a Life Orb. While you lose out on Steels, and the Grass/Posions, you hit the Fire-Types that will inevitably switch in harder. Combined with the sure chance that you have a Fire-type anyway on a Sun-Based team, and all of a sudden, a Fire-Type move dosen't seem as needed. While useable [Especially in NU, where there are almost no Steels that are used], it's not the best option in UU.
Sleep Powder is excellent for team support, and allowing Victreebel to set up, and sop it's counters on the switch-in. If your foe has a sleeping pokemon, re-setting the sun later will be even easier. Under the sun, you don't even need to predict the switch, because you'll outspeed anything common in the teir outright, even most scarfers.
So, if you're planning on running the Sun in UU, give Victreebel a try!
Victreebel @ Life Orb
Adamant Nature
252 Attack, 252 Speed, 4 HP [May need tweaking of Speed, as some may be uneeded, so can go into HP]
-Leaf Blade
-Sucker Punch
-Swords Dance
-HP Fire/Weather Ball/Sleep Powder
I've been using Tangrowth, but Victreebel is actually probably better due to Sucker Punch, as well as higher Attack and Speed. I'll definately change that on the SD team I use.
ALSO: Edited OP to show the team I'm using right now.
I've recently decided to start UU after getting bored of OU's same old same old. I'm liking the look of Sunny Day and I'm thinking Jumpluff / Tangrowth / Exeggutor / Arcanine / Cresselia / Donphan? I like the look of Specially based mix Victreebel which is probably going in Exeggutor's spot.
And also, I think three grass types is too many, so what good leads are there apart from Jumpluff that could do well on the team? Maybe Moltres, Uxie(but then there are too many psychics for my liking) or even Hariyama who can handle Chansey, Houndoom and Clefable reasonably. But I also want Stealth Rock on my lead so Donphan doesn't have to give up a move...
Oh wow. Sunny teams are becoming more popular. Heck they might bring Typhlosion into popularity!!!
Regirock and Rhyperior have great type synergy and get both set-up moves you need.
Sunny day can help cress. Victreebell has a fun time with weather ball.
Try Magmortar on a Sunny Day team, he is great.
Also with Entei getting Flare Blitz, he may be a new good candidate sweeper.
You basically need lead with Taunt to outspeed Uxie (the fastest user of Stealth Rock in UU), Sunny Day, and a way of getting Typhlosion in unscathed. Unfortunately, the speed requirement makes U-turn a poor option for getting something in without damage. It is also unfortunate that Electrode doesn't get Sunny Day, and as far as I can tell (I may have missed something) there's no other fast UU user of Taunt, Sunny Day, and Explosion. However, Persian and Purugly can use Taunt, Sunny Day, Hypnosis and U-turn. By using Hypnosis followed by U-turn, you can get Typhlosion in to launch full health Eruptions in the sun. Best yet, by switching with U-turn, you don't risk bringing him in on something he won't be able to maim or kill. The main point of this would be to weaken your opponent's best Fire resist to clear the way for a later sweeper. While Persian is usually better than Purugly, both of them get the necessary moves for this set, so Purugly's slightly higher bulk and access to Thick Fat (for Froslass and Moltres leads) might make it better in this case.
It's gimmicky, but I think it could work if you really want Typhlosion on your Sunny Day team.
That is an awesome name, why didnt i think of that? I am creating an Alt called Slashbolt right now.I'm running a Moltres similar to Slashbolt
there are a whole lot more pokemon that benefit from the rain than there are pokemon that benefit from the sunTwo small things that are better with sunny day are:
1)Variety in types (Fire AND Grass)
2)Variety in pokemon (Rain Dance has what? 8 pokemon that are good for it?)