Macro lens - expensive is better?

I would get the cheaper 100/2.8 macro non-L for the Canon - an excellent lens. Or a secondhand Tamron 90/2.5 (any variation manual or AF), or even consider any the current Sigma (horror of horrors!) macros (70mm or 150mm) which have tested exceptionally well. The Leica M lens is excellent but not even close to a true macro (as 1:1) lens. The Canon and the Zeiss you have listed are on a par one is a stop faster and one has IS (neither of which are particularly useful for macro work)

I use the lens for flowers and portraits, so I think I will settle for 100mm, as suppose other focal length.

Thanks Robin, sorry when I said I am on a "budget", I meant I don't want to buy all 3 lens and have them sitting in my drawer most of the time, but want one "good" lens that hopefully I will actually use it more often.

A macro lens is really not necessary for either of those things. Have you seen how close your other lenses focus? You may only want to add an extension tube or a close focus filter and save money and clutter from unnecessary lenses.

Leica Apo-Macro Elmarit-R 100/2.8 with Elpro 1:2-1:1 and adapter for your Canon.

The Tele-Elmar 135mm f4 has a detachable head that can be mounted on a focussing adapter or a Bellows-2 on Visoflex and Leica M9 or other M camera. For me it exceeds my once owned 65mm macro lens and cost me $400 in mint- condition a year or so ago. Maximum f stop is less important, I think, as you normally stop down a macro type lens for better depth of field. It may not be as good as the new 90mm marco lens, but I suspect the differences are not enormous as the Tele-Elmar, which is excellent at the closer range of its normal operation is not that far behind the Apo-Telyt in overall quality, and the latter cannot be adapted for short range macro use.

The Leica M is a poor choice for macro-photography. You have to make it into an awful SLR first. Plus the lenses are pricey.
So you want to go EF mount. Tamron lenses will be fine, either Adaptall 2 with a third-party (non-licensed) EOS adapter, or made in EF mount.
Of, you can add a close-up lens to an existing lens. Leica makes some superb ones with two elements, but even the one-element ones can work well.

"Macro" in the true, technical sense of 1:1 size ratio is one of the reasons why SLR cameras came to dominate the marketplace. That's why scientific photographers went so overwhelmingly to the Exaktas before other SLRs were available. To work with a Leica for this you have to use something like the Visoflex SLR adapters ( e.g., http://www.l-camera-forum.com/leica-wiki.en/index.php/Visoflex ) and, as said, you really have to be a Leitz-imprinted masochist to enjoy that.

There are a number of really good true macro lenses available in the EOS EF mount, and these include superb performers like the famous Tamron 90mm macro lens (various versions), not just Canon lenses.
Moreover, there are also lots of excellent macro lenses available in mounts that can easily be adapted for the EOS cameras, including all the famous manual focus macro lenses from Nikon and others. Even including, as noted, some Leica lenses, if you can't break the habit. ;)
MF and stop-down photography is not much of a handicap in macro work compared to the same in normal day-to-day shooting.

Oh, expensive is not necessarily better, but the very and really cheapest lenses are usually the worst.

As you mention two M cameras, one entirely manual, the other full frame digital with exposure metering and a sophisticated (almost auto everything) full frame Canon digital, it seems the first choice is to decide on these very different systems as a function of your perception of how difficult or not it is for you to work with them (If you had mentioned Leica R, the 60mm macro, or its longer Leica macro lens would have been obvious). As I don't do macro of relatively fast moving subjects (insects), the M system with its Visoflex reflex housing and bellows is fine, mounted on a tripod. I even enjoy the extra time required and some inconvenience, as it helps me to think more about why and how I am photographing my subject. And the Tele-Elmar or other lenses mounted on the Leica bellows can go more micro (a more accurate word than "macro") than simply 1:1, and if you need even more magnification, Leica has (had) small (25mm) lenses that can do that readily with this system.

I suggest that maybe it is best to first decide which of your three camera systems you want to use, given their specific macro lens use capabilities.