Hard choice. I have a feeling I'd end up going with either Whimsicott or Togepi. Or maybe even Mareep, just because sheep~ Most-likely Whimsicott, though. I love Whimsicott.
Too many choices for me! However, my main candidates would be Jynx, Spewpa, Goomy, Burmy/Wormadam, Ferrothorn, Froslass, Banette, Lileep, Shedinja, and Carbink.
Being the huge Pokemon fan that I am, I tend to have several Pokemon that I identify myself over the past 18 years of my life! I also made a list of every Pokemon from each generation in case others had already picked them for their usernames. With that said here's all the potential names that I would choose from if I were to change my forum username into the name of a Pokemon:
Not knowing much about Pokemon, I shall pick the only one that means much to me. Treeko. He was the first pokemon I ever used on the only Pokemon game I ever played.