It's really what you want to get out of your cosplay. Some people cosplay their favorite character, be it an obscure character or someone so well known you'll probably see 5 more of them. Some people want to be "the only one" at the Con. Other people WANT to be noticed and will cosplay accordingly.
I personally cosplay whoever I feel like cosplaying. At Anime Vegas 2011 I cosplayed Ken Amada from Persona 3. 2 people recognized me, but I had a blast. Anime Vegas 2012, I cosplayed Honey-senpai, and I was constantly swarmed with people. Also a blast, but in very different ways.
Ash Ketchum 1st season - Pokemon
Ken Amada Dark Hour uniform - Shin Megami Tensei: Persona 3
Kyo Sohma Winter school uniform - Fruits Basket
Honey-senpai Ouran school uniform - Ouran Highschool Host Club
Sora - KH2
Roxas - KH2
Len Kagamine Append - Vocaloid
My Life In a Nutshell