WordWeb Online

1. Of the highest quality, skill, etc.; very good or impressive
   - ace [informal], crack [informal], first-rate, super, tiptop, topnotch [informal], top-notch [informal], tops [informal], A1 [informal], excellent, first-class, fantabulous [informal], splendid, ripping [Brit, informal], mean [informal], beezer [UK, archaic], lush [Brit, informal], stellar [informal], awesome [informal], brilliant, outstanding, top-drawer

See also: ace [informal], amazeballs [informal], awesomesauce [informal], beaut [Austral, NZ, informal], blinding [Brit, slang], brill [Brit, informal], crucial [slang], cushty [Brit, informal], def [slang], fab [Brit, informal], legit [US, slang], lit [US, slang], magic [Brit, slang], nang [Brit, slang], phat [slang], rad [N. Amer, slang], radical [N. Amer, slang], righteous [US, slang], sick [N. Amer, slang], smokin' [slang], smoking [slang], socko [N. Amer, informal], stonking [Brit, informal], storming [Brit, informal], super [informal], super-duper [informal], superior, wicked [slang]