wow! top 5, hard to say - let me start with what's on my dock - these I use almost daily
mail, ical, firefox, skype, itunes, final cut, color, motion, mpeg streamclip, lightroom, stickies, iphoto, carbon copy cloner, vmware fusion, and pages
so here's my top 5 - no particular order
1) pages - LOVE THIS! I can format the heck out of some brochures/documents, etc
2) Dropbox - great for filesharing
3) Carbon Copy Cloner - backing up is a MUST
4) MPEG - it's FREE and a MILLION times better than compressor - the old compressor that is
5) final cut - love me some editing
Honorable mention - VMware Fusion - makes running quickbooks for my business a breeze