some questions

[Vulture]

None. One-race armies are pretty inferior with the exception of undeads. But those need extra effort and specific management and playstyle. If this playstyle is not for you, undead-only isn't for you. Plus, every one-race-only army will eventually face its counter-combo on the battlefield. Which, if we stay within the example of the undeads, will be a full undead opponent with no living creature in their ranks.

If you were to determine what army is "good" or "best", regardless of single-race or mixed, you always need more information like:
1. what class
2. what difficulty setting
3. whhich items do you have available
4. who is your opponent
5. what spells, talents and rage abilities do you have
6. what is your preferred playstyle
?

Only if those are answered you can begin to work on what is "best" for you. But when you're at this point you will realize that uniracial armies aren't the best choice for you ^^