It's been forever since I've used the forums, I'm surprised I actually remembered my login.
I've had a number of good games lately, but when I think of some of the most memorable, it would have to be the first time I saw certain monsters.
I remember first farming to get the Skexis fiend in Alpha--my house was completely dark, and the background on the mountain was also nighttime with lightning. Great ambiance. Took me a few tries to capture it. I would, hands down, consider that battle to be my most memorable and favorite of all time.
I remember going up against the Vampire Knight and being so upset because I was convinced it was cheating. Back then I would card/resource count the AI's plays, and it didn't make sense that it was attacking for 5 when it didn't have enough resources. Then I noticed how its health would go down or up on certain attacks. When I captured it, I found out why.
Darkon Drago certainly has a cool factor, and I'm currently trying to find and capture him, but I honestly remember being more impacted by the Soul Wyvern. I had no expectation of seeing it, then this incredibly powerful and LARGE monster showed up. I chased that Wyvern for years and only recently got it (I play more now than I ever used to, and have started actually having characters hit level 20)