Stat wise, best TD in the game is Jackson, followed by SU85 if you get them to vet 3. Firefly is actually the weakest on vet3.
Both Jackson and SU85 get insane penetration buffs, while firefly dont. On top of that vet 3 firefly takes 12 seconds to deal 480 damage (2 hits), SU-85 takes ~12 seconds to deal 480 damage (3 hits) and Jackson can deal 4 hits in 12 seconds.
Basically Fireflies are only good at alpha striking, after that they are objectively the weakest ones.
And alpha strikes are what matters most. I know the basic math behind each one points to the FF being the weakest, but in reality, who stays around after two shots in teamgames? (if you play 3v3+) Have you ever seen a chasing Jackson or SU85 in a non-hail-Mary-dive scenario? Most of the times, people tolerate 2 shots on their tanks.
And where did you get that vet3 Jackson deals 4 hits in 12 seconds with a reload time of ~ 4.6 seconds? It's 3 shots, not 4. If it didn't have the wind-up/down time, then it would deal 4 hits in 12 seconds.
So basically all deal the same amount but the FF needs less shots. So needing less shots also means less hits to bounce, but it also has lower penetration..... BUUUT it gets tulips.... So ....it all evens out. Jackson deals the most DPS with AP ammo as far as I know.
All in all: Alpha strike >>> Agility
Jackson gets the nice velocity and all....but I have extremely seldom seen Jacksons used that way in anything beyond 1v1. Most of the time, Jackson behaves like an SU85, sitting still and shooting. Better at running away though... BUT it has a large target size, especially compared to the SU85, and no chance to bounce anything.
So from my experience, and simple math, FF is the best. I really don't remember any game where my or my allied Jackson used the velocity to it's advantage in 3v3s. You usually can't chase as there are always some AT walls. High target size means getting hit most of the time. You can escape much more easily with the Jackson, than you can with the SU85 or the FF, but in reality, if your Jackson needs escaping, you're not really winning the game, are you?
In this game, in modes above 1v1, armour/firepower/alpha strike (Elite level stats) matter much more than being a speedy boy, as you have much less space to maneuver and there are plenty of f*ck more enemies