I'd recommend a Western Digital My Book, due to their connections, ease of use, styling and in my experience (touch wood), reliability.
There's a few of them now with different connections. I recently went for the 1TB Studio Edition which has USB 2.0, eSata and 2 x Firewire 800 connections (it comes with a cable that has one end as firewire 400 and so is backwards compatible).
The great thing about having 2 firewire connections is that you can daisy chain them. So I'd bought a 500gb My Book a few years ago, and now that is connected to my 1TB one via firewire. Meaning that when I connect the firewire cable to my Mac, both harddrives and all their partitions show up
The MyBooks get my vote too! I've got 3 and I got my parents a 500GB for TimeMachine on their iMac.
I've got two 500GB units and a 2x500GB dual disk. One of the 500GBs is attached to my Mac mini HTPC and the other units are connected to my iMac, the 500GB is for TimeMachine and the dual disk runs as 500GB mirror providing protected storage for my Lightroom and iTunes libraries....