Tangrowth

This is the set on my Tangie, and for me it works brilliantly. I must admit it was traded to me, and so I don't know it's EV Spread. I do know it is a Physical Wall though

Tannoyer Raylax

Tangrowth@Leftovers
Impish
252 DEF / 252 HP / 6 ATK (If someone wants to confirm them, that'd be great)

-Power Whip
-Knock Off
-Leech Seed
-Sleep Powder

This works brilliantly for me. Firstly, Sleep Powder it. Then Knock Off either it or the forced switch to really annoy the opponent. If Tangrowth doesn't need switching out, Leech Seed and drain away the sleeping opponent's health.

It's brilliant if it's the opponent's last 'mon, as the Sleep Clause can't be broken and you can have a constant drain on their health, helped along by Power Whip. If it wakes, knock it back to sleep again.

Counters
Things switching in will generally fear Sleep, but once the Sleep Clause is activated, countering becomes a lot easier. A Salamence carrying Fire Blast will usually destroy Tangrowth as it has a 4x resist to Grass.

Heatran does a pretty good job as it resists Grass and can hit back with powerful STAB Fire moves. However, it's at risk of losing its item by switching in. Starmie and other decent BoltBeamers can hit hard with Ice Beam and cripple it. Yanmega ruins it with Bug Buzz and if it switches, you get a free Speed Boost.

Bug, Fire, Poison, Ice and Flying type moves will all deal decent damage, especially if they're of the special variety.

However, Tangrowth can Knock Off some very useful items as things switch in, so a lot of counters will likely switch in unwillingly, not to mention the potential of being put to sleep if the Sleep Clause is not yet active.

I'd give examples of who's best to use those attacks. e.g. Special sweepers such as Scarfed Typhlosion can get in there and Erupt all over it and Scarfed Heatran also shrugs off Grass Knot (despite its weight) and can hit back with a hard Fire move. Starmie and other speedy Bolt Beamers have Ice Beam at their disposal and Yanmega is the perfect Pokemon to take it as it can Bug Buzz in the turn against it, and if it switches, you get a free Speed Boost.

I've basically fleshed out your ending paragraph.

The EVs are probably 252 HP / 252 DEF / 6 ATK or something like that.

Things switching in will generally fear Sleep, but once the Sleep Clause is activated, countering becomes a lot easier. A Salamence carrying Fire Blast will usually destroy Tangrowth as it has a 4x resist to Grass. The other main counters have already been mentioned.

Bug, Fire, Poison, Ice and Flying type moves will all deal decent damage, especially if they're of the special variety.

However, Tangrowth can Knock Off some very useful items as things switch in, so a lot of counters will likely switch in unwillingly, not to mention the potential of being put to sleep if the Sleep Clause is not yet active.