This is a tough one, I have been to so many great games involving Celtic but most of my memmories will probably only go back a few years until I read about a game I was at and rememberw hat the occasion meant and then that will jump to the top of my list.
I am going to try and do this in no particular order apart from the dates.
1988 - Scottish Cup final at Hampden. Celtic are 1 - 0 down to Dundee United after Kevin Gallagher scored but Frank McAvennie was on hand to score two lates ones for the Hoops and that was the double sealed in our Dentenary season. I was only 7 at this point and I remember the excitment of the game. Itw as a right family day out I remember being with my Da, Granda, Uncles, Cousins, and my Granda's mates from his work. It was a crazy day.
Early 1990's the year escapes me I am thinking 1992 possibly, I had went to Cologne for my first ecer European away game and it was a bad experience Celti where trailing 2 - 0 when we got them back to Parkhead. The atmosphere at CP was electric, Tiger Tim was in tremendous form the crowd was bouncing there was one guy stood next to me handcuffed to a copper, he was obviously out for good behaviour or something. It was a fantastic performance from Celtic in particular from John Collins and Gerry Crainey. We managed to overcome the 2 - 0 deficit and win 3 - 0 but we got humped in the next round.
Again my memory is a bit hazy with the dates and without googling I remember we where 2 -1 down to Partizan Belgrade and we brought them back to Celtic Park and humped them 5 -4 we went out on that silly away goals rule due to come comical defending from Derek Whyte. Our polish superstar and playboy Darius (Jackie) Dzieknowski scored 4 in an unbelievable game and they where crackers as well.
Anfield in the run up to Seville, me and my cousin went down to Anfield with a bus load of soccer hooligans most of them where my mates my cousin was shitting himself with the antics that where going on, on that bus. It turned outw e where the only 2 on the bus with a ticket for the game althoughw e where in the Liverpool end. My cousin was so scared that he hid his ticekt in his show fearing one of the lads would have stolen it from him

Anyway we gets to Anfield and thanks to John Hartson and Alan Thompson we had another memorable game to look foward too.
Think Martin O'Neil first old firm game and 8 goals and you have it!!
I don't really want to give reasons for my worse games as they just bring back bad memmories.
Any defeat agaisnt Rangers!!!
Uefa Cup final against Porto in Seville
Loosing the league against Motherwell last day of the season.
Defeat to Inverness Calley Thistle in Scottish Cup.
Loosing Cup final to Raith Rovers at Ibrox.