The Pentax MX is all manual, quiet and smooth and quite reliable; slightly smaller than the OM-1; shutter speed and aperture are visible in a finder the size of the OM-1's. The ME Super goes to 1/2000, has a slightly smaller (but excellent) finder, is reliable too.
The LX is a pro-level camera, tough and refined like the pro-level Nikons, Canons, etc. 1/2000 top speed, mechanical between 1/75 and 1/2000. Meters to EV -6.5. Big, very clear and contrasty finder, excellent interchangeable finder system. About the size and weight of the Nikon FM/FE series.
If you can, try out the different recommendations. Find what you like best. All have some great lenses. I have Pentaxes and Nikons; I like the feel and ergonomics of either, but prefer the Pentaxes. I also love the Olympus OM series and their ergononics, especially the OM-1; I could have gone with Olympus instead and been satisfied, except for the lack of interchangeable finders.