It's hard to pinpoint exactly which "part" I like best. The iMac's overall engineering-- the elegant all-in-one hardware (as mentioned by themoonisdown09) teamed with an intuitive, unobtrusive OS-- gives me more time than ever before to actually do activities with my computer beyond maintaining the computer itself. I can finally "ignore the man behind the curtain", so to speak.
The first two or three weeks I owned my iMac, I repeatedly found myself saying, "Okay, it's time to update antivirus and/or defrag-- Oh. Wait. Nevermind." For me, and for many switchers, it's a paradigm shift.