In terms of the different reactors in general, the most useful ones would be completely depend ON the game you're playing. The player's responsibility is to choose which reactor is the most useful for each specific situation and each specific game. Tough choices are what make good strategy games. In theory, all the choices of Reactors would be viable in different situations (sort of like the Orbital Command Stations are now). I get the feeling that if this debate between us were about the "new" Orbital Stations before they had been implemented, you would be just as against it since you seem to have a very conservative attitude towards change - but look how positively that change affected the game.
Given that I play Fallen Spire, Normal waves don't do that much next to the EXO waves of golems and hunter killers.
You tend to play your games in a vacuum, and that's fine, but since you make your games into the exact same scenario everytime (Fallen Spire), the same Reactor(s) are more likely to be useful to you. However, for players who play with more variety, the choices would be much more interesting.
Runner: Pointless. Average speed is too slow, it will just be caught by fast ships or snipers. I like my reactors were they go up, and the Command station can't move so...
Your argument seems to be - "I wouldn't use this, so it's pointless". Honestly though, I think your attitude is very disingenuous. Most players probably don't use any more than half of the bonus ship types, does that mean the other half they don't like using are worthless?
Your playstyle is not the only one in existence, and shouldn't be treated as such.
The Runner could be used for players who enjoy micromanaging their fleet (I know they're out there) to split to the enemy fleet into two groups, thus taking pressure off of the Orbital Command and giving the player extra time to respond. It could also be used to run in the event that the Force Field covering the Orbital Command is about to die. It could persist on the death of the Orbital, as long as the planet is in supply.
Phantom: Just dies with the command station anyway...
Make it persist on Command Station Death like The Runner, as long as the planet is in supply.
Trap: Galaxy wide EMP, well nice, is unlikely to be that useful over an EMP warhead. I doubt I be able to do more more then attack one system when it blows, but the fact that it would effect the AI homeworlds might be a little useful.
Planet-wide EMP when it's destroyed (typo).
Tank: Reactors are not meant as Decoys. They are support structures, and should be built in the backlines, not in the front lines when you can avoid it.
That's what makes the idea ingenious. Though it doesn't produce much energy, the AI doesn't know it's a decoy, so it spends a significant amount of time trying to destroy it, giving you time to respond.
Special Beam Cannon, I might use these but for some reason I don't think they will be that much better then using a turret and a reactor.
Turrets have a limit, you can build a new Reactor on every planet.
The Turtle: What exactly would you be protecting with a shield around a reactor? Another reactor?
Some extra protection for your Orbital Command or another key-asset on the planet.
The idea is to add more strategy to the choice of Reactor. Maybe it wouldn't affect your conservative and repetitive playstyle, but for people who enjoy new and unique challenges every game, the Reactors would be just as (or maybe more) important of a choice as Orbital Command type. Just my 2 cents, I think you improve strategy games by adding more strategy.