Not by personal preferance but by stats moveset
First of all, TORTERRA RULES IN ALL STATS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! HE IS MY 2ND FAVORITE STARTER(behind Swampert!). Now, to answer you question, I would say Venasaur because it ties with Meganium and Torterra with the lowest total stat of all the starters. It has terrible attacks. Its highest power attacks are solarbeam and double edge which both have TERRIBLE drawbacks. It has low attack and this is bad because most of its attacks are physical.
well there is none, that may sound cliche but it's the truth your pokemon's strength relies on it's trainer, it's nature and of course it's move set, so what if you have a strong pokemon but you don't know how train it well and does'nt know what moves are good with it, and what about meganium not knowing any good attack, well if you know meganium is my favorite grass pokemon starter, but i'm not saying this without proof, meganium can learn leaf storm which is a good grass-type move if you know how that is(i can teach how if you like) the best move for grass-type pokemon is:solar beam or grass knot,leaf storm(but only use this if your sure to use it)energy ball and synthesis or alternate type move well you see what i'm saying is that there is no weak pokemon, it all depends on how you train it
I would say meganium is the worst because it does not have many good moves it's best attack is solar beam but it's stats are alright.