What a fun thread, you should make one for each age.
If playing MA Ermor avoid MA Marignon at all costs.
If playing MA Ulm avoid MA Bander Log at all costs.
MA Mictlan is always a pain no matter who your playing.
If you come across MA Agartha try to destroy them as early as possible, they are a powerhouse the longer the game goes on.
MA Man struggles against MA Arco especially later on.
Errm that about it for me, I still have lots of races to play and fight...
I believe the game overall in a particular age is balanced, as every nation has at least 1 other nation that can just walk all over it fairly easily. I appear to be in a minority believing this to be true however, virtually everyone else thinks Dom3 is unbalanced in some way.
Velusion for example above mentions MA Vanheim, yet in the very MA game he himself hosts containing every nation, MA Vanheim was one of the first nations destroyed.