Venusaur and Charizard

I would power up VW-SB-Venusaur, as it is useable in almost every gym to slize through the vaporeons, as it has the perfect attacking moveset. In addition to this it is already big and does not need so much further candy and dust.

[Edit]
as it is already at level 30, you may just want to keep it as it is and power up somebody completely different.
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I'd say you don't really need to power up any of these mons any further. However, if you have dust to spare and need more/stronger grass attackers, I'd spend dust on either of the Venusaurs. If you're already set with good grass attackers, just leave them as they are, as they should already be performing great.

Charizard is not the best fire attacker with Ember, so save your dust for better pokemons/movesets.

I'd go with VW/SBeam if you really want to power up, but i think it's already a good attacker at lvl30.

Just FYI, SB is a horrible abbreviation. Venusaur can have both Solar Beam and Sludge Bomb.

Vine Whip+Solar Beam is significantly better than Vine Whip+Sludge Bomb for an attacker.

Razor Leaf+Sludge Bomb has better type coverage than Razor Leaf+Solar Beam for a defender.