WordWeb Online

1. Of the highest quality, skill, etc.; very good or impressive
   - ace [informal], A-one [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
    
2. Brilliant; very good
   - socko [N. Amer, informal], righteous [US, slang], fab [Brit, informal], crucial [slang], rad [N. Amer, slang], radical [N. Amer, slang], nang [Brit, slang], phat [slang], legit [US, slang], awesomesauce [informal], amazeballs [informal], beaut [Austral, NZ, informal], lit [US, slang], storming [Brit, informal], blinding [Brit, slang], stonking [Brit, informal], brill [Brit, informal], wicked [slang], cushty [Brit, informal], ace [informal], def [slang], magic [Brit, slang], smokin' [slang], smoking [slang], super [informal], sick [N. Amer, slang]

See also: super-duper [informal], superior