The sealed bottom bracket (like the UN54 that [MENTION=112025]wolfchild[/MENTION] mentioned) is at the top of the list in my opinion. I was re-packing them several times a year, replaced with the sealed BB and haven't thought about it since.

Sealed hubs on the wheels are the next. It's not that hard to service and kind of zen in a mindless way, but why?

After that, maintaining and replacing the chain (instead of letting it go) keeps you from having to replace the cassette as frequently.

I agree that derailleurs don't need that much attention, but still the bike falls over and it gets banged up or the hanger is bent, the pulleys get gunked up and need cleaning, cables and housings need replacing, shifters can malfunction or break. For truly low-maintenance of course the single speed or fixed gear.