Personally I have to say two times. As I always primarily played a warrior the enti's sword glitch had me playing hours a day. Even though it was a pretty harsh exploit it was still undeniably fun watching rends tick for 150 (not sure of the real number).
A more specific time period would be ruin the months before the migration and then probably the year to follow the migration. From the time time prodigy was just establishing its self and promoting a lot of the migration to ruin, and then the following year and few months where three premades in a week wasn't too surprising.
Oh hell yeah, enti's was a lot of fun.
Another period of time I enjoyed was exploiting the shaman windfury enchant.
And when I had money I enjoyed sporting my naxx pads, I felt like a star.
skirmishes against the pwntage brothers, obliviate, no mercy, deathsstone, mythos, pk, shockstar, goma, very memorable.
retaliation dueling ladder (placed 1st baby)
countless duels with seep, az, hiredhit
any random warsong against Merryjr, Bobgnarly, Goma, rogue Pwntage (miss playing against this guy, always a trip), Fleas, or no mercy members... BATTLE OF THE TITANS..
I very much enjoyed playing with the guild Mind Yo Manas. heavycream, az, push, clairee, and bob were fun to play with, and quite the characters. Smashing NO MERCY with Mind Yo Manas in under 12 minutes was my most memorable moment of twinking. I literally yelled "YEAHHHHHHHHHH" with all of the power of my lunges along with everyone else in vent who was just as happy and proud of our guild tag at that moment as I was (maybe the happiest I have ever been). It surpassed my memories of playing against Pizza Hut with the Cyclone A-Team and returning the flag, though it is a good one. Az aka Shaggy was the best 2's teammate I could ask for, and a good friend to me.
I have to add the days in vanilla grinding sergeant titles, warsong reputation tabards, rumsey rum, and fishing hats with Azebec al ne Adan with my warrior Dstroid. Whenever I want to feel nostalgic I hope the guild armory is always there
http://us.battle.net/wow/en/guild/haomarush/Azebac_al_ne_Adan/roster?sort=rank&dir=a - it's like a guild locked in time, it always makes me happy to see it.
This makes me sad.
(many edits, i just keep wanting to add more as it comes back to me)