to some extent its not a question of formation as it is of your own battlefield decisions. no matter what formation you put the AI in, if you manuever in all the right ways you can tear apart any formation, isolate bits of the line, etc. you can help things with formation, but a big part is to force yourself not to use exploits and play it straight up.

Couple of examples: with Darthmod, when Darth would demonstrate the effectiveness of the formations, it was always with his army stationary, or nearly so. This helped the AI figure out what to do. Playing in a way similar to this makes battles more challenging, but it also takes out some of the generalship.
with any of these mods, if the AI is positioned in defense on high ground, it is almost always possible to get them to keep rotating until you've gotten equal or higher ground. the AI doesn't know how to assess that its losing its advantage and move into attack mode. It eventually figures it out, and sometimes it runs away, sometimes it moves to another spot, and occasionally it attacks.

I personally like to let the AI maximize by having a big battle in the plain. Perhaps that's because I'm a better recruiter than I am general. And that doesn't apply if I'm using the Getai. If I'm using them I ambush every chance I get.

And as for formations that are great, does anyone else have an astounding love/hate relationship with the steppe cav formation? Its awesome, so I love it, but it kicks my butt and takes names, so I hate it.