Red, due to being severly underleveled (highest level pokemon was 63, 2nd was 59). But favorite hardest battle would be Green/Blue (not as underleveled by as much...but still really challenging).
__________________
Japanese Soul Silver FC: 3438-2493-8839 (most trades, game beaten)
English Soul Silver FC: 1763-4978-7462 (use if I say so, game not beaten)
PM or VM if you wish to battle, keep in mind I don't have an OU team set up yet, so I may still be painfully easy to face.