Honestly, I think SoulSilver (specifically SoulSilver for one major reason) is at the top along with D/P, if we exclude BW2 hard mode, then it's either D/P or SoulSilver. Why Soulsilver you may ask? Because if you decide to play with legendaries, which many people do, Ho-oh can be caught very early game in Heartgold, (around the 4th/5th gym if you get lucky with a catch) but you can only get Lugia in SoulSilver until right before the 7th or 8th gym, where it's practically too late for the main story, and the elite four has a very strong team this time around. Yes, you can get a level 70 ho-oh before Red, but his team is so amazingly put together, it will still probably get decimated by Red, who is the hardest trainer in any game, by far. Cynthia puts up a challenge too, especially in D/P. So if we exclude BW2 hard mode I'd say it goes to Soulsilver or D/P.
