Yeah, I definitely think gentoo is the hardest distribution all-around, I used to use it from stage1, but broke it, and from experience, the best thing about it is the package manager, it's hard to keep updated, hard to install.
Mepis is one of the best distributions, I used it for almost a week

, I really liked mepis.
Slackware is different, it requires a bit more configuring, but I think it's worth it.