For me, I have a few.
For one, when I first started playing Mario Kart Wii. I remember missing a few homework assignments for a couple of days because I was just so hooked.
Next, the first Xat chat I went to was Brawlerjesse/Nin Jesse's chat, back when he first made it. I ended up being best friends with him, and good friends with Angel.
I had a great experience playing in clans back then, especially when joining my first clan, which I think was Ec2. I remember fanboying over Ec2 Zigzag, especially when I had him in my friend roster. I also fanboyed a lot when playing in races with Mr Bean, Mr Mean, DN Yoshi, Yoshiller, and other people. Also, I used to love being in Mario kart Wii videos. I geeked out when I was in a couple of Yoshiller's and Mr Bean's videos. I uses to be in some videos with a person named "Michiru", and I was in a Skype call that was also uploaded to YouTube and I geeked out.
Those were all great experiences, but the most recent and the best experience I've had was playing in a worldwide with Zachy, Rachy, and Aqua on a Saturday night. I already considered joining 4DR, but the final nail on the coffin was this worldwide. remember when I first joined I was like "Oh, some 4DR members, cool!" Then, I raced and had an absolute blast. The best race, one that which gave me an adrenaline rush, and one that made me decide to join after, was one in N64 Jungle Parkway. It was the start of lap 2, and I had Rachy and Aqua directly in front of me, and Zachy behind me. We were just in one pack for the entire lap, until we eventually got separated. During that lap I was like "Ohhhh this is getting intense. I'm around EACH 4DR member dude!" That was great. Then, me asking to go on trial and being added to the Skype chat. I instantly felt at home. You guys were great to me, and I was super excited when I found out I passed the trial. I hope I continue to have good times in this clan in the future