The choices are fewer than they might seem. Listing RQ1, RQ2 and RQG is a bit redundant - effectively there is no RQ1 (it is the same as RQ2), and RQ2 & RQG are close enough that almost anyone who was a fan of RQ2 would be perfectly at home with RQG - certainly that is the bulk of the audience for RQG.
Mythras is not really a fork of RQ6 - it
is RuneQuest 6, just re-named, and the book got a new layout, functionally the rules are identical. RuneQuest 6 itself is a fork of Mongoose RuneQuest II, written by the same authors. There are people who play in Glorantha with RQ6/Mythras (I've run a few games like that myself) Hannu of
Notes From Pavis and the Mythras Ecnounter Gen
Mythras Encounter Generator is one. There is a lot of support on his blog for Mythras-in-Glorantha. There was a copy of "Adventures in Glorantha" (for Mythras) printed as a preview for Gencon 2015. I may know where to get a copy from if you are interested - it was never on sale, and can't be after Chaosium dropped RQ6 as the system for the next edition.
Support-wise RQG is a no-brainer as it is the only Glorantha game in town at the moment. The system makes me groan though. It was great when I played it (80s) but RQ6/Mythras did a great job updating the system I can't go back to RQ2. Even though there are bits of Mythras I think are too much (like turn cycles, reach), it is still much more interesting that RQ2/RQG. You can of course pretty easily adapt RQG material to Mythras or even OpenQuest, with a bit of work.
Shame about 13th Age Glorantha, it looks good but it got dropped - ie. there are no and there will be no supplement support for it.