Picking just one is difficult. There's the general success of SMSE, not just because of how popular it ended up being, but also because it was the first time I've really made any sort of full game. I'd attempted a few things in my time using SMBX (and Mario Builder and such), but the only one that really got finished was a 10-level thing I made in my first week of using SMBX back in December 2009. It was terrible and I didn't save it, but I do wish I still had it to look back on. With everything else I attempted, my design skills were still less refined and I'd feel like I drastically improved over the course of a single world, which feels bad for any given project as a whole. I'd also lose inspiration quickly and cancel things, so at one point I decided not to attempt another big project until I was reasonably certain that my quality and motivation are both consistent enough for that to work. Eventually, I started work on SMSE when all I really had at the beginning was an idea for 1-1 and making a point to myself not to cancel it just because I'm demotivated. After working steadily and being patient with myself for about a year, I actually finished the entire thing. I'd finally made my very own game that I could be proud of.
At the risk of sounding even more arrogant, I'd say my general popularity as a level designer also counts as a memorable part of my experience here. I think I just came at a good time, where Knux's forum was all but dead due to TrekWeb, so I didn't have a lot of competition in terms of level attention. If I was a new user right now, there's no way I'd develop such a reputation, as there are plenty of other designers among us now that are exceptionally good. It was also pretty fortunate that TrekWeb's judges were fairly active in reviewing my submissions, too. I used what I'd learned as a lurker of the original forums, playing those levels and developing a general sense of quality, and nervously put out my first public levels for people to see. My first try at TrekWeb (Castle of Machines, if anyone remembers that) was rejected due to difficulty, which is why Mechanical Castle (if anyone remembers THAT) was made. That was accepted, which meant a lot to me because I felt like I was beginning to fit in with a group of like-minded Mario fans that know what they're talking about and have a similar passion for good level design. I remember lurking Redigit's forums (followed by Knux's, after that died) for a good several months before I finally worked up the courage to register and say hi, so the reward of having my work recognized positively was that much greater.
I also had the privilege of helping out the community in other ways. After starting to review levels as a non-judge once I learned that was okay, I was offered a Level Judge position, which is the first time I was given any sort of Internet responsibility and I was able to handle it. I'd also gotten to be a part of several collaborative projects and official contests, both as a contributor of levels and a selected judge. It means that I do a good enough job that people want me helping out with big community events that a lot of people get excited for. Admittedly, I could have done MUCH better with Revisited, as that was my first real experience with a collaborative game thing. I wasn't as perceptive at finding bugs as I am now, and it could have been released in a slightly better state if not for the fact that I found TONS of things in the final testing. I recall the general response to that enormous list being along the lines of "Hell no, this is way too much to fix", followed by it being pushed out to the public in the state that it was. I don't blame anyone for that, as I wouldn't have made such a huge list in the first place if I was better at finding stuff from the start. But I've learned from that, and I feel that I generally did a lot better with many of the other things I had the privilege to help test. Being able to help with big-name projects like Apocalypse of Foroze, Mario Classic, and New Great Castle Adventure (among others) was a great honor, as I looked up to the people involved.
Yeah, this got a lot longer than intended, but these are the experiences that come to mind when looking at the topic.