For me Bolton. Even though I was immensely confident going into that match, that we would win, as others have said the actual realisation, that barring a highly improbable GD swing, we were there; the years of being so close to extinction; the squad being somewhat more grounded and nearer to you average everyday fan, that the multimillionaire players lack; the barely credible results en route and the sheer pleasure of seeing what a threat we were going forward, with goals from all over the pitch. Having scouts from other clubs sat next to me over those few seasons leading up to that game and enjoying asking them how they would deal with our right-wing in the first half, then how would they deal with our left-wing in the 2nd half.
Forest was of course, immensely enjoyable, cloud 9 stuff but I didn't expect anything less than a victory and turned up at DC so confident promotion was sealed before a ball was kicked, that the final whistle was almost an anti-climax (we did only win 1-0 after all)
I've seen every relegation and promotion. Bolton will always stand out.