What is the toughest transmission I can run with my sr20det in my 200? I've done a little research but I haven't come up with too much.

A p11/b15 or spec V tranny, though the spec V will require a lot to drop in (ie: p12 bracket and belhousing, spec V spindles, axels, shifter & linkage, and a p12/s15 dual mass fly then you get the option of 240mm or 250mm clutch). 'Course, the odds of finding a p11/b15 tranny are slim to none.

Or you can get a b13 tranny, shotpeen and cryo the stacks, and make sure you get the third gear taken care of before welding her up.

Wrecked B15's/P11s are fairly common, though the LSD trans is a relatively rare option. Still, it's the best choice if you have the patience to find one.