2001 and 2002 and here's why.....
The cast pistons have much tighter tolerances. They came in all stock '00 to '02 gen 2 vipers. (I've found that I don't burn any oil at all on my '02, but I did on both my earlier year GTS's.)
ABS, Stronger frame, Bigger brake calipers, No neutral gear rattle, More intake and exhaust cam lift, bigger (10 quart) oil pan with baffles...... just to mention a few more reasons to save for the 2001's or 2002's
The older non-ABS vipers are still good cars. You can get the frame re-welded, upgrade the small radiator fan, modify the oil pan, change the fill grade weight on the tranny, and pay around $7,000 for ABS and you can pretend you have a 2001 or 2002.
Every year has some advantage over other years, I just feel that the '01 and '02s have the most to offer in stock trim.