Download speed is linked to CPU speed and data through-put. Hence the Z3 X-Surf 100 + AmiTCP 4.0 gets the best numbers, but getting there on an A1200 would require a tower set up, and I'd just go with an A3000 or A4000. Sneaker-net may be best for non-surfing situations (large file downloads).
Only there is no zorro3 for 1200 towers

well except that buggy one micronik? made
for the most part everyone with 1200 expansions would be limited to z2 mode of the x-surf 100.