The great thing about AmigaOS, is there's plenty to choose from!
The things that really make Workbench nice to use are Visual Prefs - you can spend hours fine tuning your desktop for a truly individual look, ReqAttack for the lovely animated system requesters and Picasso96 for displaying on a Voodoo card! MagicMenu is another essential too.
Other programs that get my vote are Birdie, Directory Opus, Delitracker, AmigaAmp, AmiNet Radio, Photofolio and noclick. Without noclick, my Amiga would have driven me insane by now!
:-D