Aho!: means the same thing as baka, but has a softer sound...
Tako: also means idiot.
Mou gaman dekinai : I can't stand it!
Shimatta: Shit!
Kuso: Crap, Damn
Kobito: Shrimp
(not to mix up with koibito: lover)
Dasai: Strange, uncool
ecchi, etchi: pervert (not really slang, but...)
Hentai: Pervert or Porno, etc.
Sukebei: Sex fiend, Pervert
Kakkoii: Hot, Cool, Hot(like as in GUYS. haha)
and if anyone says something mean like this to you:
Ijiwaru: You're mean!!, Cruel~
Kouhei ni shiro: Play fair! (No fair!)
Mou takusan da!!!: That's enough!!!
Usotsuki!: Liar!
Urusai!: Shut up!
Damare!: Shut up!
or even:
Sore dake?: Is that all?
Ochitsuite: Calm down