Canon 60mm Macro is EF-S, will not work on 6D.
Canon MP-E 65mm Macro will work but is a super high magnification macro lens (1:1 to 5:1). I'd not recommend it for general purpose macro shooting or for a first macro lens. It's rather difficult to use and is manual focus only.
IMO the best bang for your buck is the 100/2.8 USM (not the L). Just get a tripod mount (B) for it. That and the lens hood are sold separately, so the price in the end is a little higher than the lens alone.
Image quality is practically indistinguishible with the 100L IS. So basically you just get IS for the extra $500 or so. And yet much of the time it's better to be on a tripod or at least a monopod with macro shooting, anyway.... So even with the 100L you'll probably still want a tripod mount (D), which is a bit more expensive than the one for the non-L (if you get the Canon branded tripod mounts... there are cheaper third party clones of each)
The Tamron SP 90/2.8 is a pretty nice macro lens, too. There actually are two versions.... the older one is the cheaper. They recently introduced a second, upgraded premium model with VC and USD added (which are Tamron's versions of stabilization and USM-style focus). Of the two, I'd probably consider the newer model largely for the USD focus (which will be considerably faster than the micro motor in the older, less expensive lens). Oh, and by the way, the new version does support full time manual focus override, same as the Canon. The older, cheaper version (which apparently will continue to be offered at a lower price point) doesn't have FTM and tends to be slower focusing. The 90mm f2.8 VC USD is priced somewhere in-between the two Canon 100mm models.
The Sigma 105/2.8 OS HSM is also a fine lens. OS is their version of stabilization and HSM is similar to Canon USM focus drive.
AFAIK, the Tamron and the Sigma cannot be fitted with a tripod mounting ring, which would sort of be a deal killer for me. I don't know if any of them have Focus Limiters, either, which both the Canon 100mm do. Used right, a Focus Limiter can make a lens faster focusing, particularly for non-macro uses. I also am not sure if the Tamron or Sigma are all Internal Focus, like the Canon.
There is also a Tokina 100/2.8 AT-X Macro lens that offers excellent image quality. However, it also cannot be fitted with a tripod ring, doesn't have a Focus Limiter, and doesn't offer USM style focusing, so cannot do FTM (focus clutch is used to turn off AF and enable manual focus).