I kinda like all of them.
In terms of power, the Templar is the clear winner. Each battle he starts out kinda like a shitty ranger, but after a single rend he becomes a monster that just gets stronger and stronger. At 3 rend, you've basically won. I think you need to level him up early as most of his cool shit is high level. Seriously, the guy can passively reflect attacks, can get bladestorm, etc. It's nuts.
The Skirmisher is like a jack-of-all-trades. At high levels, the guy is ultra mobile, just zipping around the map and killing shit all over the place. He has so damn many mobility skills. He can kinda fulfill any role I think and is generally nice to have no matter what the circumstance. He can do a ton of damage if he wants, or he can... *gasp* skirmish.
The Reaper is really useful for scouting. The massively decreased detection range is so damn useful. Not much good in a fight though, but they can do some damage. I mostly just use the claymore (after the upgrade). I like bringing them on missions where I don't start in stealth. Just send them flying off to the front and scout like half the map in one go.
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Incidentally, my most powerful "hero" characters aren't any of those. I gave my personal character and his bond so many skills they can fight the entire world. I invested 200 AP in both of them.
I actually don't recommend that. While it's fun to have a supercharacter, if you give someone too many skills they can't actually use all of them so it's quite a waste. Not to mention with fatigue you can't even use them too often anyway.