Well, I have always liked Tangela since I saw it, and with its new evolution now I can use it competitively. However, I'm still unsure of which moveset I can give it.

This is the moveset I had planned:

Tangrowth @ Leftovers
Impish | Chlorophyll
186 Sp. Def, 122 HP, 202 Def
~Power Whip
~Brick Break
~Amnesia
~Leech Seed

The idea is to use Amnesia for enduring Special Attacks.
Brick Break is there because is the only good Physical TM available for me. I would give it EQ or Stone Edge. And I would like to keep at least 2 offensive moves.

Would this moveset work? Do you have any suggestion for Tangrowth?

I recently saw one used very well in an online tourney, I think this was the moveset, but I can't be 100% certain

Tangrowth w/leftovers
Knock Off
Sleep Powder
Leech Seed
Ancient Power

The thing was a beast, which would suggest HP/Def EVs, and was more of an annoyance set. It would be easy for you to create the same thing, but I doubt Tangrowth can be as good an offensive threat as some of the other options available. Personally I feel Sleep Powder is indispensable to setting up w/ Amnesia.

I see. So if I want to keep both Leech Seed and Amnesia, I would have to sacrifice an offensive move for adding Sleep Powder, but that would reduce Tangrowth's type coverage

I was thinking that I could make a pure Def Tangrowth (129 Atk, 129 HP and 252 Def EV's) and take out Amnesia, but I don't know if it would actually work because it will be severely damaged (if not OKOed) by Flamethrowers and Air Slashes. What can I do?

well a pure offensive tangrowth has 252 HP 252 Defense 6 Attack/special attack
it would know

Tangrowth@Leftovers
Impish/Bold
252 HP 252 Defense 6 Attack/Special Attack
Sleep Powder/Substitute
Stun Spore
Leech Seed
Power Whip/ Giga Drain

this set is abusive and very annoying...a tangrowth does with double powder...put on pokemon asleep then they switch out, and not to break sleep clause you use stun spore paired with leech seed...the bold and impish nature depends on what move you would want power whip/ giga drain....power whip is for a more offensive purpose, while giga drain will allow tangrowth to survive longer, with leech seed and leftovers...for it resisting special moves...it really cant do that effectively...that's exactly like asking rhyperior to resist special moves(which he can do), but both arent going to survive that long against a special move...maybe last 2 hits, but by that time you would be rendered useless...so just pair him with a special wall

I actually played Tangrowth in this weekend's tournament with this setup

Tangrowth@Leftovers
Impish Nature
Evs - 252 HP 252 Def 6 Atk
Power Whip
Earthquake
Sleep Powder
Swords Dance

It was phenomenal, against teams with physical sweepers, Tangy was an excellent swicth-in (provided they didn't have a Fire move). Powder them to sleep, Swords Dance on the switch and beat down. I generally reserved Tangrowth for the end of battles, to wear down the opponent and scout for Fire attacks.

Even when they have Fire attacks, the HP/Def EV combination can shrug one off if Sleep Powder hits.

You should know our local tournaments do not use the Sleep Clause, so this version may be less effective in areas that do.

The funniest thing was that no one knew what I was up to. One guy decided not to switch after his poke was powdered, and I got in a second SD! 700+ Atk Tangrowth FTW!! It was great!

I see. Increasing Tangrowth's Sp. Def is not really recommendable as it doesn't have a high base Sp. Def to start with, and that means I can take out Amnesia and replace it with Sleep powder instead.

So, for me, this would be the most suitable moveset for my Tangrowth:

Tangrowth @ Leftovers
Impish | Chlorophyll
252 HP, 252 Def, 6 Atk
~Power Whip
~Aerial Ace
~Sleep Powder
~Leech Seed

I will replace Brick Break for Aerial Ace so Tangrowth will cover more types.

Thanks for your suggestion

Hey, no prob, Tangy is pretty fun to use!

Impish@leftovers
252hp/252def/6att
Ability doesn't really matter
in a battle because no one these
days evens uses sunny day or has
a sunny day team.
Leech Seed
Power Whip
Sleep Powder
Knock Off

Tangrowth is known as a physical sponge and a leech seed drainer. Trust me this is the best way to run your Tangrowth because I tried and it works great.

'best' is such an shaky word when referring to pokemon builds. While I like Leech Seed, and draining strategies, it was cool simply beating down with Tang at the end of the fight.