Level 100 Statistics (see level 5, 50, 100)
Tangela was never really that mighty, but it would still be a hell of a lot better if that Special split did not happen. But still, you can get some use out of this Pokémon yet. That Special Attack is still great, and with that boosted attack power, it is sure to do some good damage, and be a good part of your UU team.
This is a highly predictable set for Tangela, but it works well enough, and really, it doesn't learn enough moves to diversify a bit. But Tangela is still a cool Pokémon that can do very well in the UU metagame, if you time its use right. Sunny Day does three things for Tangela: doubles its Speed (it should now be faster than any other UU Pokémon, providing that they don't get the boost too, like Jumpluff), allows SolarBeam to be used without recharge, and increases the power of Hidden Power Fire. Should you face a Pokémon that is resistant to these attacks, like a Dragon or Fire type, you can put it to Sleep and let another Pokémon take care of it later. Leftovers is usually the item of choice for Tangela to let it take Earthquakes and such better, but if it gets paralyzed it ruins this whole set up, so Lum Berry is always there for consideration.
Leech Seed and Ingrain can keep it in the game for a while. Reflect is also there if you want to use Tangela this way. You could use Growth to bump up that good Special Attack even more. Stun Spore is there if you want to do more than just put one Pokémon to Sleep.
Max out Special Attack, and go with Modest nature. 60 Speed EVs guarantees you will be faster than Timid Electabuzz and Manectric after a Sunny Day, meaning only other Chlorophyll Pokémon, Jolly Persian, and Electrode will be faster than you.
Checks and Counters
That Special Defense is simply horrible, and Sunny Day will boost the attack power of Fire attacks, one of Tangela's weaknesses. Also, there are other Grass-types that depend on Chlorophyll for the Speed boost, and also carry Hidden Power Fire. Be wary of them, and anything that uses Ice Beam. Its physical weaknesses aren't as big of an issue, but it will still die in a few powerful hits, so be careful.