I think the "best" or "worst" characters are highly situational; the game mode, selected map, roster of the opposing team and the skill level of the players both on your team and the opposite team makes a huge difference in what character might survive better or worse.
In the current Mystery Portal event, Rando's Revege (Vanquish Confirmed), I've had some really large vanquish streaks when the game has randomly spawned me as a Sunflower and the opposite team is mostly "average" players. By finding a decent semi-long range vantage point with a steady stream of Zombies approaching it's pretty easy to drop a Rainbow Heal Flower, snap your Heal Beam to it and start blasting away. The Sunflower's high accuracy means that the longer range has little effect on your shot placement and you can hit consistently while many of the Zombies who might return fire will miss a percentage of their shots due to the range involved and having your Heal Flower at hand will remedy the damage that you do take. On a map with a fair number of choke points, like the Colizeum, and with the Electric Sunflower variant you can do a lot of damage very quickly and survive well if you can keep the opposition from closing the range.
By the same token I've had horrible experiences trying to play the Camo Cactus in the same game mode because the Camo Cactus needs longer ranges and a decent guarantee that enemies won't randomly spawn at their back or flanks to surprise them when the Cactus' peripheral vision is limited while scoping in on long range targets.
The biggest variable is who you're up against, how good they are and what their favorite character might be and how well either team functions together.
A few weeks back I was in a Turf Takeover game with a highly skilled pair of Players; one had obviously invested hours and hours of practice into Captain Cannon and he had a knowledgeable partner playing the Astronaut by his side at all times with his Heal Beam of Science engaged to keep the Cap'n alive and vanquishing. The pairing made it really difficult to survive if you didn't keep their position in mind at all times and they were difficult to counter if you didn't have the right character and experience. More than 95 percent of the Captain Cannons which I've encountered have been terrible at worst and average at best but this was a guy who had invested the time and effort and because he had teammates actively supporting and protecting him he was easily the top performer every game.
