I am at the point where my world is large enough to attack other masters. So, whom should I attack out of these guys- Beleth, Doh-gar, Oumm. Any advantages or disadvantages of attacking the various parties? Thanks for the info.

Do you intend to go for the "There can be only one!" ending? If not, you may be best off to avoid attacking anyone until you have some other reason to do so. (If you're pursuing the Keys of Truth, for example, you'll want to keep Doh-Gor around until a certain conversation points you to him as a target.)

You can get from Doh Gor what gives him strength, and then you can give it to Beleth, and I assume you can take it away from him, but I have not tried it. So I'd go for Doh-Gor before Beleth.
Oumm isn't special, I think, I always attacked him, because he never said much. That's how I usually decide who goes, the ones without dialogue.