We're big fans of Gallery here. It works well for most needs and seems simple enough for everyone to use.
Jacob - WebOnce Technologies - 30 Day 100% Satisfaction Guarantee - Over 5 Years Going Strong! Website Hosting, PHP4&5, RoR, MySQL 5.0, Reseller Hosting, Development, and Designs
From a standpoint of simplicity, I've always been quick to recommend Snaps! to people looking for a single-user gallery that's fast, easy to integrate/skin and very efficiently written. I've found it takes little effort to customize code within the application too.