I'd go for someone who can compliment some things that chaos can't do. Either swarming, or skimming. So d-eldar, or orkz depending on what your army does. If you're more CC, go with orks, if you're more balanced/shooty, deldar should be a great help.
Well, I used to use some deldar that my friend gave me (1 squad and 2 raiders) to start out with my chaos, so I'm cool being allies with them (my friend plays 4000pt dark angels with unholy amounts of armor). But then I think of all the other cool races D=
Just about anything can be allied. What "looks" nice is thing such as Imperial Guard (Renegade's) and SM's (Fallen).
A suggestion from someone who has played a fair number of Apoc games:
Spend about an hour the week before the game, creating your own reference sheet. For example, first game we played, I had CSM, Imperial Guard and SM equipment on the table, being controlled by myself. Problem is you end up with 5 books you need to reference: Apoc book, 40K rule book, and the 3 Codex books for the 3 different armies.
Since then I made my own refernce sheet (I retyped it out) listing only the equipment you are going to be fielding. i.e. I don't need to list Typhus if I'm not going to be using him. You then only have 1 sheet of reference paper, and can refer to the books if it's really an issue, not if you're doing small things like looking up someone BS. You could even cut and paste with a photo copy machine, if you really have to.
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