Wow, what a question... and a difficult one.
As a pretty new member who's gotten to really enjoy some large sitewide events (such as Clackvent), I have a lot of fond memories of GH and have learned to love this place and that it's pretty damn awesome.
So, I guess my favorite GH thing ever is actually... a series of 3 things.
My entire GH journey began when a GH member (who shall temporarily remain nameless), sold me an Arctic Lion Keyng V2 at cost. He saw, on /r/mk, that I had really wanted a Keyng from the recent Binge sale and that I had missed the boat and just wasn't fast enough. So, he decided to help me out

. That was my first example of GH generosity. This single key spiraled me out into a full on artisan collector. Suddenly, I loved Clacks, and BB's, and Gasmasks (which I had previously thought looked silly). It was a huge change.
That change to a love of artisans then brought me to actually try GH. My first posts here are in the Hot Keys Talk thread, I do believe, and the main reason I initially came here was for info on martin's sale and to try and get in. When I noticed that there was a community around just discussing the sale and lamenting over missing it, I decided to stay and participate. This meant I was more active and interested in GH when my second event started: the Ctrl-Alt Advent Giveaway.
The CA Advent Giveaway was something I was eligible to enter, so it immediately piqued my interest. I initially started by just entering every day and not reading other posts. But, slowly, I started reading all of the entry posts, because they always contained
something good. A nice thanks, some good holiday cheer, or just some optimism. It was all very refreshing. Everyone was remembered and had more of a personality than they did on /r/mk, this was when I started noticing the areas where GH beats reddit, people are just more connected, it's a real community, the knowledge base is crazy on this site, and everyone here is so generous, it blows /r/mk out of the water. As that event progressed, I looked forward to not only seeing who won (and their thank you message to Bun), but also very excitedly waiting for the next day's prize. The whole event was a lot of fun and was a good insight into how most people on GH, are just nice people. And then December ended... and we all know what happened next...
Clackvent started. Clackvent is my actual favorite GH experience. I was quite eligible to participate and, after Clack removed the "first post wins" rule, I had a very real chance of entering each day, so I started obsessing. This was when I found Clack Therapy and started engaging with more members and just having more conversations on GH in general. God, Therapy was so fun for those two months, so many good times to be had. Everything from the gnome jokes, to the heartfelt messages we all sent out when we learned Clack had been hospitalized

. That was where I got to learn so much more about the site, the well known members, everyone's personality, and... a tiny bit of drama or two. That thread was amazing and was one of my favorite GH things ever. Entering the event was also amazing. The rush of excitement whenever you saw Clack post in Spy or got a Tapatalk notification and then that ******* clenching feeling as you waited for the winner to be announced. I, obviously, entered to win, but never expected to
actually win. My first win was on JD-day, so another show of GH generosity with CC giving almost everyone a Clack. After winning one, I certainly didn't expect to win more... let alone, make it into the actual orgy

. So, Clackvent was a lot of fun (I hope he repeats it next year!).
After Clackvent ended, I had made a friend or two, found some ICs and GBs I was interested in, and found some great discussion threads. I was now a GH'r. /r/mk now rarely gets opened and, with it, my reddit usage has gone down,

).
Sorry for the wall of text guys :-/, I am pretty guilty of doing that really often. But, my favorite GH experience was
joining GH, so I felt it appropriate to tell the whole story of how I got here and why I decided to stay. I love this place and I have to say that it has one of the greatest communities on the internet

.
Thanks for the giveaway hwood!
Another example of GH generosity! WHEN DOES IT END!?!?!?!