I have done very well with my Kislev Rhordia, and I have several friends that play KoM / Rhordia that also do well overall. I run cavalry heavy, a friend runs pure melee infantry heavy and the other runs infantry heavy with a good amount of shooting.
KoM, League and Brotherhood are all a bit tougher to play than many of the more fantasy heavy armies, but they are close enough in power level to be competitive with a good strategy / tactics. A bonus with playing human armies is that you can blame it when you lose
It is hard to give generic list advice, since almost all units are fully playable. If you give us an idea of what kind of units you want to use, or what kind of theme you want, preferably with a draft list, it is easier to help.
I can give the generic advice that the following units are good and fairly easy to use:
- Human wizards. They are cheap, the KoM ones come with lightning bolt, and drain life is ace with infantry hordes. Bane chant is very neccessary for humans because of our low CS.
- Volley guns (helblaster). Consistent damage dealing for a cheap price. Canīt go wrong with 1-3 of these guys. Bring the next unit to keep them around longer.
- Battle shrine (war altar). Brings both good shooting, a scoring unit and the +2 nerve bubble that keeps your chaff and warmachines around for an annoyingly long time.
- Honor guard (demi gryphs). One of the best melee damage dealers in the game. Give them haste potion to increase their threat range and they will treat you good.
Human infantry, cavalry, heroes and monsters are all also fully usable, so there are a ton of options for good lists.