http://www.solomid.net/guides.php is the best place for (most) guides, in my opinion. If you choose a "featured" guide, 90% of them are written by a top ELO player, and they go into explanations on their mastery choices, rune options, item purchases, etc. I used to use mobafire until I realized just how bad most of them were (haven't taken a look at the one Arch linked though, so no comment on that).