When I first joined I wanted 'Theoneyoufear' but there wasn't enough room, so it was 'Theoneyoufea' xD After that I chose Pikachuiiiii.
Because Pikachu the 5th, I don't know. I was just really desperate for the name Pikachu and someone took it and never used it :L FFFFFFFFFFF

^
l
l
right here

Probably the most unused name I have ever seen is "Gaaic," which I contributed to one of my friends (so, technically, it's my name xD). A few other names I have created that no one else has used:

Snapdiamond
Brutality
Radkela
Kaptka
Kounoichi (actually a rip-off from Naruto, after "Konoichi")
wolfbeat (this one is very old)

and, finally, Wolfwithred

Other usernames that I thought were pretty unique

Sick
Adouchebag
Humanmotor
Anlapolo (used to have an extra "a")
and a few others I can't remember