Which one of these pre built companies are the most reliable?

Any company that tells you the exact model of all the parts they use should be OK. That way you can go for quality parts. Any company that doesn't tell you has something to hide.

I've had two CyberPowers over the years and both worked as advertised and held up very well.

I think each one had some minor thing wrong out of the box (slightly bent grill on one, can't remember on the other) that I fixed myself.

Tech support can be a little hard to understand at times.

Alienware is Dell = crap on reliability.
Alienware is also EXTREMELY overpriced.

DO NOT BUY ALIENWARE!!!

MSI and ASUS are also good, but IDK that they make full towers. I know their laptop's are good though.