Gun turret: Gun turrets have good damage, when kept up-to-date with upgrades and ammunition, but they suffer from their low health and range. A small increase to the range (to 22 or so) could help them to make better use of their damage, compensate their health a bit and make it a bit easier to cover one's base early, when resources are rare.
Laser turret: They are good at everything, with no serious drawbacks. Decreasing their range a bit (to 20 or something) could help making them feel a bit less like masters-of-almost-all-trades.
Flamethrower turrets: I stand by my opinion that combining perfect automated threat identification protocolls with terrible aim is embarassing for whoever builds these things. It almost hurts me to watch them. One would have to adjust the numbers, but I'm sure that's not too much of a problem. I favor a strong reduction of the sticker damage (being on fire from getting touched by the stream) to reduce the "touch of death" property of flamethrowers.
As a proposition:
-No contact damage
-Sticker damage to 52/s and duration to 15s
-Ground damage to 26/s, base duration to 15s, maximum duration also 15s (repeated shooting just refreshes the duration and the damage multiplier)
-Takes longer to reach the maximum damage multiplier for ground fire (maybe 2s continuous fire)
-Make the damage multiplier decay exponentially: per frame: dmg -> dmg * 0.something
I also stand by my opinion that it would be very enjoyable if robots wouldn't kill themselves by flying into the flames while trying to make repairs. Fire resistance would be the easiest solution.
Note: I made a mod where you can test these changes: https://mods.factorio.com/mod/MudcrabempireBalanceMod
-The mod also contains some changes to the tank and the PLD, feel free to disable them if you don't want that (you'll have to cut the respective lines from the "item-final-fixes" file).
Following, my original post:
Note: This list is probably too extensive and may need to be boiled down for actual implementation.
Gun turret: The one and only. A small increase to the range (to 20 or 22 maybe) would be nice but I guess being the first available and the cheapest will relegate this turret to an eternal early-game role. Good ol' days. Sniff.
Flamethrower turret: Combining perfect threat identification protocols with terrible aiming algorithms should give any programmer a serious headache. I propose increasing the fire-pool damage, reducing the direct damage, maybe increasing the fire-pool size and duration a little and giving the damn thing perfect aim already. This, combined with the biter's lack of fire resistance would make the Flamethrower turret better at being an aoe support turret which is what I think this guy does best.
Laser turret: Easy to supply, lots of health, un-resistable damage and good range. This guy is basically a jack-of-all-trades. I propose to keep this theme, but making the Laser turret weaker overall (reducing damage and range, mostly), thereby making the Laser turret a good line-holder and gap-filler, but still requiring other, more specialized/powerfull turrets to deal with larger threats.
Laser turret corollary: Adopt the same idea for the PLD to encourage the usage of weapons, even later in the game.
Artillery turret/wagon: These two are fine.
Grenade turret: A grenade lobber on a turret. I'm not sure about this one, but it would roughly be an early option to the Flamethrower turret, more heavily oriented towards aoe damage. Large potential for collateral damage (especially when using cluster grenades) would make this turret a bit hard to use (at least it would definitely need a minimum range).
Rocket turret: The go-to option for dealing with the big guys. Optionally, make this turret prioritize the biggest enemy in range, instead of the closest.
Cannon turret: Mounting a cannon on a turret sounds way too awesome for me to not do it. It would be similar in size and defenses to the artillery turret. Obviously the strongest, but also the most expensive one. I guess there's a risk that this turret becomes the new "best" turret, but with it's high price and supply requirements I think that is valid. Also, for dealing with less contested areas or for dealing with very large waves, other turrets should still be good.
Corollary: I don't know wheter it is intended, but when I use Flamethrower turrets I often run into the situation that the biters reach a structure (walls, mostly), leaving a damaged wall with a fire-pool next to it, and when my robots fly in to repair the wall, they die in the flames. It's very annoying and this issue increases with increasing research (robot speed and fire damage). Again, I don't know if this was intended, but if not it would be very pleasant if robots wouldn't take damage from fire-pools (they fly above it?).