Assuming we're talking about the Voigtlander lenses (which are the only real options in LTM at these focal lengths) I'll second what everyone else is saying: the 15mm Super Wide Heliar is a fantastic lens, but that focal length is ludicrous. It's not an every-day lens for most people, but the 21mm Color Skopar is much more manageable in most circumstances.
That said, the most impressive thing about the Super Wide Heliar to me is how good it is edge-to-edge and how little distortion it has for such a wide lens. You'll typically get some smudging at the corners, but if you're careful about your composition, it can be difficult to even tell you're using such a wide lens:
Both of these were taken with the 15mm Super Wide Heliar and if I hadn't taken notes, I'd probably think they were something much longer. But, of course, if you're not careful - or if you want to play with such things on purpose - you can end up with weird perspective distortions with the 15mm that you really don't get with the 21mm:
So... yeah. It's a lens I've had for a long time and don't use as much as I should, but I'm always impressed with it when I do. It's a fantastic lens, but you have to get ridiculously close to whatever it is you want to photograph with it.
Either way, whether you get the 15mm Super Wide Heliar or the 21mm Color Skopar, make sure there's absolutely no "haze" in it - these can't have the haze cleaned out; it's usually permanent - and make sure they come with the matching viewfinder they were originally sold with. I've seen a few crop up missing the viewfinder when people have upgraded to later M-mount ones and kept the viewfinder, but they should be a complete set.