As for me, a 250mm f/4 or f/5.6 with an MFD of 75 cm or larger would be the best. But an EF-S 60mm f/2.8 IS USM with at least 2 stops stabilization would also be good.
I'm not really interested either in built-in ring light or larger than f/2.8 aperture as I'm used to take my shots at f/8 or f/9 and sometimes – if the subject is relatively big and I can have a larger working distance – at f/5.6 or so.
I don't think that focus stacking is really that important as the larger the magnification you have to use the smaller the subject is. I have a front-view shot of a small weevil – about 3 mm – and with the EF-S 60 mm and the 25 mm extension tube I have almost the complete beetle in focus with f/8 while it almost completely fills the frame (the photo).
As for the definitions: 10:1 or larger is micro. Between 1:1 and 10:1 it is macro. From 1:2 to 1:1 you can call it close-up. Smaller than 1:2 is just a regular photo with something small in it. It's not macro and not even close up.
And at last just a theoretical question: would a macro zoom lens with at least 1:1 magnification ration in any focal range be possible? Because a RF 60-180mm (or 70-200mm) f/2.8 IS USM macro only – from 2.5:1 to 1:1 magnification ratio – would really be unique.