This is very hard sice it changes all the time for me. Right now its Range vipers but my top 5 favorite are

#1 Crimson Guard(85) (need more)
#2 Range Vipers(90)
#3 Vipers(86) (I'm all set with them)
#4 Eels(85) (wouldn't mind getting a few more)
#5 Alley Vipers(89) (just a couple more and i'd be set)

Cobra Freak