I normally don't use nicknames, some Nicknames I use often include "Neko", "Xandra", and "Ch'marr" - I"m not too creative with nicknames, I don't use them much.
My Liepard in both Black 2, and White is "Neko" Japanese for cat, male or female its always the same nickname
Male Watchog in White I call him "Ch'marr" (comics are offline since long ago, but around 2002 I used to read a comic about an anthro cheetah named Ch'marr)
Probably the worst nickname would have to be my Meowth in FireRed "Chimuelo" and "Bobo" to my Slowpoke.
and in Sapphire I named my Zigzagoon/Linnone "Duvalín" (which is a Mexican candy) and "Mazapán" (another Mexican candy) to my Skitty which I never evolved