I think Pandora Hearts is pretty popular, but maybe that's just because I spend my time with PH cosplayers.
I need to agree with Last Exile and Black Cat, two of my favorite series, which just aren't watched as much as I think they should be.
My Kira, it's so good, and I've only found one person that's watched it...well, I don't know that many people, so I guess that's not weird.
I was obsessed with it, and it's not as little known as it could be, but so few people seem to love SKET Dance anymore, though it's just awesome.
Cosplays: Brock (Pokemon) 95%, L (Death Note) 100%, Kurapica (Hunter X Hunter) 100%, Bikky (FAKE) 40%, Count D (Pet Shop Of Horrors) 70% Dave Strider (Homestuck) 95%
New York Comic Con 2012:
Friday: Sober! Gamzee Makara (Homestuck) 80%
Saturday: Mituna Captor (Homestuck) 80%
Sunday: Mituna Captor (Homestuck) 80%