The best moment for me is when a youth player finally has finished the training you envisioned. I still remember the key players from my time in Hattrick. Josip Dumont-Damien was one of the first ones, Luc de Romrée de Vichenet and Mamadou Toure are players I remember fondly from my last generation.
The Hattrick community is colourful, it really helps expand the Hattrick world. You can find plenty of helpful managers that can help you with Hattrick and real life questions.
Some of my Hattrick friends I've known for more than a decade now, which is longer than quite a few of my real-world friendships. You build up trusting relationships with them, and meet people you'd normally never meet. That's also what I really enjoy about the federations in Hattrick - some are really places where you can share your personal stories. A federation in Hattrick was one of the first places where I disclosed I was going to become a father!
What I like about the Hattrick community is that there are real people and you don't just compete against the computers. You play against each other, but you can also help each other in the forum and talk about everything.
I would say the community is the strongest reason I have stayed in Hattrick for so long. The game too of course. After having trained my players and played in the French Division 2 for a while, I started focusing on training international players to help the Moroccan team. Today 80% of the defense of the Atlas Lions was trained by me!
My biggest achievement in Hattrick so far is that I have won some titles with my second team in Bahrain and reached the semifinals of the National Cup in Italy, but I hope, a few months from now, that it will be winning the World Cup for the Italian NT.