Tazrael: You know, that's a really tough question to answer, with a lot of possible responses. It's hard for me to pick down a particular favorite given that I have a certain attachment or relationship with many different cards for many different reasons. But how about instead I give you a few highlights:
The first time I'd ever traded cards with someone, it was back in... '95 or so, or maybe '96, it's hard to remember. But I remember they wanted my Ball Lightning: which at the time was probably an aggro powerhouse.
Then there's Yavimaya Enchantress, which is important to me because it's the first time I looked at a card and went "...hold on, you could make a deck around this ability, couldn't you?"
MechaPony: oh man... so I'm assuming you mean somewhere where the fandom didn't stick more than any lack of fondness or aptitude for the character, right?
Sadly that probably goes to my most recent attempt. For whatever reason, while I love and resonate with the character, the big picture just didn't fall into place for me as far as fandom went.
It made me realize I was basically a casual fan trying to keep up with people who had far more of a dedicated passion to Vivzie's Inferno. And as a result, it just kind of... ended up sliding off. Which is a shame, because it's not like I didn't enjoy playing him!
But that might be... selective. Because there were a lot of parts where Tamers just kinda dragged on and/or got really weird and it's hard to detach my memories of it from the bottled-lightning surrounding fandom and presence it had.