Really, all Chlorophyll mons we have seem to out class Victreebel in some way.
But that's not really saying much. I'm sure I could find some way that Victreebel "outclasses" each of those sweepers in their niche. More accurately, I could find some aspect of Victreebel that makes it somewhat reasonable to consider it over each of them.
Actually, I think I'll do just that.
Venusaur is a better special sweeper.
Victreebel has Weather Ball, opening up the Hidden Power slot for another coverage type. It also has Giga Drain, which is often preferable to Energy Ball.
Sawsbuck is a better physical sweeper.
Victreebel has more Attack, Power Whip, and Sucker Punch. If you are trying to get rid of Subs/Sturdy, it also has Bullet Seed as an option.
Shifty is a better mixed sweeper.
Better attacking stats, Weather Ball, and a better physical Grass STAB.
Lilli has Sleep Powder and Quiver Dance.
Victreebel has Sleep Powder as well. No Quiver Dance, but it can still boost SpA and SpD with Growth and Stockpile if you really need them. Speed isn't exactly a major factor when you already have the Chlorophyll boost.
Most importantly, Victreebel has much better coverage, meaning that it can take better advantage of its boosts by being harder to wall.
Tangrowth is bulky as crap.
Victreebel has a higher SpD, Stockpile, and Synthesis. While it has crappy overall defenses out of the box, a boost or two makes it semi-respectable.
Jumpluff is the fastest pokemon with Sleep Powder under the sun.
Most of that speed is wasted, though. Victreebel is better able to take advantage of the fast Sleep Powder.
Vic really doesn't have anything going for him, it's better to save him for UU.
Except that he isn't really viable in UU without Drought. Also, it's not like writing an OU analysis for him somehow prevents people from writing a UU one.
Edit: Just to clarify, I'm mostly playing devil's advocate and don't really think that Victreebel should replace Tangrowth on a team - it isn't exactly a physical wall. But as a Special or Mixed Sweeper, it isn't strictly outclassed by anything, even if Venusaur/Shiftry are generally preferable.