I'm a prefold/cover fan on the whole, but for out and about I really like BumGenius 3.0's. I think that they're very easy for babysitters and such (except my MIL--velcro is apparently far too overwhelming for her). I love that they come with two inserts at no additional expense (for overnights and longer car trips). I also think that they fit very trimly and very nicely under clothes, especially for the slightly older babies. I probably only have about 8 BG's, but that's been more than enough for us so far. I've also been very fortunate that I've gotten most of mine as seconds, so they've been extremely affordable.
There are a few others I'd really like to try, like the FB One Size and the GroBaby, but I'm not in a hurry, maybe if I find something on sale.
Candace - Mama to Molly (02/06), Max (06/08) and Maggie (03/10)