I've found Mandrake to be the easiest to install so far... not that Ubuntu was difficult, but there was a question on partitioning which a novice trying to dual-boot might struggle with. I could imagine them inadvertantly wiping their Windows partition when installing Ubuntu, if they don't pay attention and just click without double-checking.
The Mandrake install was even easier than the Ubuntu one... all the times I've installed it, it's correctly picked-up the correct settings and the default options have all been correct. It's a bit prettier than the Ubuntu installation too
The Red Hat install I used a few years ago (think it was Red Hat 8... can't remember now) was good too, but haven't really used Red Hat nor Fedora for a long time now.
Am gonna give Gentoo a go soon, but from what I've heard the installation is a real biatch cause it's targetted specifically for your environment/purposes.