Simple and easy to clean. Those fancy ones that recline with soft padded covers can be a nightmare as food gets in the crevices.
We have a padded one and it is such a pain to clean. I end up taking it outside once a week to hose it down and scrub it, but in summer it takes all day to dry and in winter it takes forever.
The constant wiping/scrubbing/etc has ripped up all the padding after what must be almost two years of use (but we're out of the house a lot so it hasn't had really heavy use). If I ever planned to have any more children, I'd end up having to buy another highchair rather than passing this one down. As it is, DD is now just about grown out of it and it is so skanky that I wouldn't even give it away to charity - it's only fit for throwing out.