If you're allowed to feel nostalgic about things you never experienced the first time round then I'd have to say the original Asayan audition episodes as it's just wonderful to watch history in the making.
But since I only got into MM/HP in 2002 I'll say the first performance of Shabondama on 27HourTV. 6th gen hadn't done anything before that and everyone was speculating on what part they'd have to play in the next single, and seeing that performance of Shabondama I think Reina far surpassed everyone's expectations.