Not sure I could vote in the above poll - because many of the products are designed to do very different things.
DayLite - this is a great program, with a very responsive development team (I've been using it off and on since it was beta). The software is great for those in the sales business, has it's own Palm syncing conduit, comes with free bulk-emailing software, has great mail merging capabilities, an ACT! importer, etc.
That said, the calendar lacks basic functionality in terms of flexible repeating appointments, and changing single instances of a repeating appointment.
More importantly, the interface has become a bit bloated and obscure.
I know MarketCircle is planning to address both issues in the 2.0 version of the software, so I hold out hope that with that release this really will be far and away the best piece of software out there.
In the meantime, I use Address Book and iCal - they work well together, are easily used by the rest of the system, and you can write great Address Book plug-ins in Applescript that will automatically type/address envelopes or letters for you. Both iCal and Address Book can be improved, (and they will be with Tiger) but for now, they serve my relatively basic needs very well.