Let's go through it step by step:
Gen 1: all three of these guys have had pretty excellent runs. While initially the weakest, however, Venusaur has emerged as the king of Sun teams, while Charizard suffered from Sneaky stones. Blastoise has always been in the middle. Still, all three are high quality in either design or usefulness.
Gen 2: Here we go. Physical/Special split hurt Typlosion pretty hard, and Meganium has never been good in Singles. Feraligatr has seen a lot of use, however, and is pretty badass.
Gen 3: Blaziken has been consistently awesome and swampert has seen lots of use. Sceptile, however, doesn't see as much use, and other pokemon fill his niches better.
Gen 4: Infernape was a favorite in his gen, and probably sees a lot of use still (Although Blaziken overtook him once Speed Boost released). Torterra is a little hard to use well, but his bulky and hits hard. Empoleon, except for his typing, isn't SO useful compared to other water types. No one in this gen is terrible.
Gen V: Emboar is a little too slow to really see competitive use, but he's got an interesting design and isn't completely useless. Serperior is pretty great. Bulky + Fast is a great combination, and if Whimsicott didn't exist, Serperior would be my favorite subseed candidate. Samurott is... Okay I guess? Not terrible. Not great. Pretty average.
I contend that if we're talking about ALL battle types including Doubles and triples, Meganium gains a lot more truck and it's either Typhlosion or Sceptile's game to lose. If we don't count double/triples, then Meganium is the worst of the worst by far, no contest.
