Unranked ----> C- Rank / C Rank
Mega Venusaur is quite an underrated Pokemon in Ubers and it's really not hard to see why, honestly. It has lackluster stats across the board and doesn't appear to have any defined niche that allows it to be usable whatsoever. However, upon further inspection, you can see how Mega Venusaur has numerous positive traits. Its ability, Thick Fat, mitigates two of its weaknesses, leaving it with only weaknesses to Flying- and Psychic-type moves. Its typing is rather uncommon and gives it many unique resistances including Fighting-, Water-, Grass-, Electric-, and Fairy-type resistances. These resistances allow it to take on Xerneas and Kyogre well, which is quite a feat. It can also abuse Chlorophyll pre-Mega Evolving which can come in handy at times, although I wouldn't count on it really. Mega Venusaur also has a recovery option in Synthesis and a solid movepool to back it all up.
However, Mega Venusaur isn't particularly bulky, nor it particularly strong, it's also not very fast, and it is pretty mediocre. It's outclassed by many Pokemon like Arceus-Grass, Ferrothorn, and Arceus-Poison, for the most part, and a plethora of other defensive Pokemon. Not to mention that Synthesis is cut down in Rain, and the omnipresence of Kyogre means that Rain really isn't all that hard to come by. Mega Venusaur is overwhelmed by a lot of Pokemon and strong attackers in general. The biggest flaw, really, is that it takes up a mega slot which could be used for practically any other Mega in the tier. Despite these flaws, it does have some select pros. It also fits the C Rank description very well:
Reserved for Pokemon with viable offensive or defensive capability. They have certain niches or utility that allow them to perform in specific roles. Reliant on fairly large amounts of team support to function and minimize their inherent flaws. They may also suffer significantly from opportunity cost to the point of being mostly outclassed.
It fits this description. For these reasons, I'd like to see Mega Venusaur put into either C- or C Rank.