Well, it really depends what they need but most folks want a website they can 'control' to some ability and with lot's of built in features. As you said there are many CMS' out there. I'd say pick one which appeals to you, maybe one which has a separate template system, since you're a designer, you can make a nice front end, that is all they will care about anyway. (with template scaffolding this should speed up development time).
I know all the php/wordpress snobs on /. will dismiss this and laugh but personally if i'm building a site for someone (usually for no money and limited time) I just install wordpress, 'secure it', then use or modify a theme. Just basic stuff, you can remove the meta links from the front page and other tweaks and now they have fully functioning site that you don't have to do much to. If you are hosting it, be prepared to apply security patches the instant they come out and backup the db.