I put my favorite as Victreebell > Vileplume > Venasaur for the Kanto trio; Amoonguss and Roserade add other types of combos, but I digress.
Vileplume has the capability to wonderfully combo all the effects that sunny weather can provide, more so than Venasaur and even Vileplume at least. But Victribell has the best of the best and is perfect in a Sunny Day. Keep in mind that sunny day can become a Doublade, and hurt you with the fire types, but here is a few nice things to keep in mind when dealing with Victreebell on your team:
The 'Solaris' Build:
Ability:
Chlorophyll (don't bother with gluttony)
Moves:
Synthesis,
Solar Beam,
Growth,
Sunny Day
As many of you know,
Sunny Day can cause differences in many playing fields, boosting
Fire Type moves and a few secondary pieces that come into effect when it's in play.
Growth, which normally only boosts
Attack and
Special Attack one stage each, while under the effects of
Sunlight,
Growth ups the user's
Attack and
Special Attack by
two stages. This combo itself is lofty enough to stand on it's own freeing up potential in several Special type and Physical type moves, such as
Acid Spray,
Powerwhip,
Gigadrain,
Energy Ball and many others.
What the Solaris build can really do is utilize
Solarbeam and turn it into a laser cannon of doom with enough sunlight to cut through many different types of Pokemon with ease. The major drawback in this build is that it has little defence against a few types that it has little effect on and opposing weather effects; but the Solaris build is a 'high risk, high reward' and when properly utilized, can be used effectively.
Growth can even offset some of the type resistances when used in conjunction with
Solarbeam.
Synthesis is just another move that utilizes
Sunny Day to the fullest, restoring 2/3 of HP over 1/2 regularly. This is more of a personal preference, and could be subbed for another move without too many repercussions.
Acid Spray would be extremely handy here when faced with
Flying,
Fire,
Bug,
Dragon or other
Grass Types to further offset
Solarbeam's ineffectiveness against those types.
Yeah it blew my mind too. I didn't think it had a hand in Victreebel's line of work, I just thought it was a flower.
It's speed holds it back a lot from being as competitive as the others. It has pretty good Special Attack and gets the typical Grass/Poison-type movepool. It also gets Dazzling Gleam as an extra option for Special Attacking. Victreebel's has the added option of being a mixed attacker and gets Leaf Blade, one of my favorite moves. Venusaur has bulk on its side more than the others and is I believe the fastest of the three. So Vileplume would be able to keep up if it didn't have an abysmal 50 speed.
Venasaur is faster than the others (base 80) yes, but isn't much faster than Victreebell (base 70), and falls short when in Sunlight due to Chlorophyll being Victreebell's default ability. While it may not learn moves that are super effective against Venasaur, it's higher Attack stats can utilize Wringout and Slam against Venasaur effectively with a Sunny Day/Growth build without many worries.