The original B.A.T. was a figure that just made sense to me. Instead of a faceless army of goons for the Joes to beat up, why not a faceless army of unthinking machines. Brilliant!
The B.A.T. has a very specific look, one that is not easy to duplicate. I considered quite a few different minis before selecting the Star Wars Mini: Republic Commando - Scorch.
Scorch has two things going for it to make it B.A.T.-like: it has the T-shape visor, and the backpack with those two posts sticking up. I put a little green stuff on one to make a crude claw, leaving the other as-is to stand in for the laser attachment.
Of course, the best thing would be to pick up an original B.A.T. micro fig, but they tend to sell on eBay for a little more than I want to spend.