Been thinking but it's so difficult as I've enjoyed different eras for different reasons and some of my favourite years supporting United were when we were crap
I'll rule out the 70s and early 80s as I used to go with my Dad and just enjoyed games as a day out rather than understanding the implications of the 'season'.
Mid 80s to early 90s were my favourite 'times' supporting United (despite it still being the tail end of Liverpool's era) because it was me and my mates going every week home and away on the trains. Yes there was the violence and the fact football fans were treated like third rate citizens but it was a combination of my age plus the fun and adventure of it all that made it special. Add to that Mexico 86 and Italy 90 with England and it just escapsulates my whole informative footballing years I guess.
2 seasons stand out for me though in terms of favourite United seasons.
92/93
Growing up between Liverpool and Manchester - those 27 years without a title were hell for me and my family and made this Championship winning year very special. Having come so close the season before against Leeds made it all the more sweet. It felt so good and I'd waited so long for it that when I finally 'came down' I almost wished we hadn't won it and still had it to look forward to.
At 23 years and counting for Liverpool, and given his age, this why I understand Joe's troubled mind.
98/99
Despite the doubles and titles and double doubles or whatever they call them that followed, I remained the biggest pessimist in the world when it came to United. Whilst those around me got more and more excited by the possibility of the Treble, I just saw the potential for disappointment growing larger and larger as those last few crazy weeks played out. God knows how but it all panned out but part of me hopes never to have to go through all that again.