The custom that I view as my first "professional" level custom is my 26th completed custom, Snow White Queen. She is based on the song of the same name by Evanescence. She was made for the "music theme" custom contest at the Memphis MLP Fair in 2007. I really took my time with her, chose the design, worked on her over a few months. Three years later I'm still really happy with how she turned out.
I see this custom as a turning point. I began to take my time with customs again. I worked hard on designing them before I started. I worked hard on the execution of my designs. I paid more attention to my painting technique, trying to create smoother finishes with my paint and attempting smaller and smaller details. She's also the first custom on which I used Dollyhair nylon hair, the same material used on the original ponies. And I really started to see customs as a viable form of artistic expression. Oddly enough, this is also when I started pumping out more customs and becoming more confident in the finished products.
Snow White Queen came in 2nd place in the judged category and 3rd place in the people's choice category of her contest.