Canon's L-series where almost every lens is optically excellent.
You wouldn't think so to read some of the rants on DPR .... but that really just goes to show that for any given subject, there is always an internet site where people will mouth off at great length and from a great height.
Then again, lots of Canon people wind up with an ugly "L Fetish", and spend their days walking around fantasising that there is a secret substance called "L Glass" which, kind of like the special sauce on a big mac, does magical things when you poke a camera through it. Complete twats, the whole lot of them.
Anyway, the moral of my little story is to say don't get to hung up on the name or model number on the lens, let alone on whether it has a red ring or not. I've got a fair collection of lenses, about half of them with the red ring, the other half without. Yes, my best lens is an L, and my worst lens isn't, but if we follow good standard statistical practice and discard the outliers, the picture is much less clear. In order of quality (reinstating the outliers), they go:
1: L
2: non-L
3: non-L
4: L
5: L
6: L
7: non-L
8: non-L (now sold)
Those two non-L lenses near the top, by the way Dave, are both EF-S units. Yes, there is some point to staying away from EF-S lenses if you are thinking you will go full-frame before too long, but if you do you will miss out on some absolutely brilliant glass. Converesly, an "L" badge isn't by any means a guarantee of perfection. My 24-105 distorts horribly at the wide end: it's as bad at 24mm as the 18-55 is at 18mm, and the 18-55 costs maybe US$100, where the 24-105 is something like US$1400. Go figure.