Stout diff.
 

Which diffs are the strongest among the 2nd gens? Gear ratios? Will any 2nd gen diff swap? How about 3rd gen diffs, will they swap in a 2nd? Any stronger? How much torque can they take in stock form? Axles? Joints? What's gonna break is what I'm getting at. Haha.

Read the FAQ, lots of this is covered there. TII = 8" diff, which is stronger than the NA 7" one. The S4's with an LSD have a clutch type, which will most certainly be worn and in need of a rebuild, but will be the strongest diff. The S5's with an LSD have a viscous type where the special fluid (not replacable, not the gear oil) will certainly have broken down and it won't be especially effective either. In the NA diffs, you can swap the Torsen from a 1.8L Miata (1994+), and for the TII diffs, you can swap the FD Torsen. Both involve swapping the working piece into the FC housing.