I think what you're really asking is what kind of actuator can I get that's powerful, inexpensive, and simple, which rules out hydraulics.
I'm also building a robot arm... first to catch balls and hopefully fencing!
So I'm interested in speed and power, but not precision which is what most robots aim for.
To the best of my knowledge, if you want high power and use commodity parts, the answer is RC car motors and planetary gearboxes.
- Motor: you need to get a sensored motor for precise running (especially during startup), braking, and counting revolutions. Here's an excellent recommendation:
Very high amount of torque in a compact size due to being an outrunner (likely the only sensored outrunner out there thanks to John Holmes)
- Planetary gearbox:
Look at BaneBots' P60 and P80 gearboxes.
I have used both of these to soup up a Scorbot 3 that I bought at a surplus depot and am impressed.