I've GMed and played RQ2, RQ3, Stormbringer, and Hawkmoon. On balance I prefer the greater simplicity of RQ2 over RQ3. I disliked RQ3 Sorcery and Shamanism and the Fatigue system, while eminently resonable in theory was too much of a pain to calculate for every NPC or to track in combat.
Stormbringer and Hawkmoon are fine as long as you don't want everyone to have magic and to cast spells. As everyone a spell caster was one of the features I liked about Runequest (and I'd prefer the next campaign I run to be one where all the PCs can cast magic), I won't pick Stormbringer/BRP. If you want a divide between fighters and non magic users, then Stormbringer/BRP is probably the way to go.
I only have a couple of Mongoose products, but they seem rather slip shod or lacking in play testing. So I wouldn't use that.
Mythras seems like it is designed to appeal to people who are already experienced with Runequest and who now want more rules. As I age and as my main player group evolves into a more casual, less rules oriented group less, rather than more, rules is my preference so I don't see any appeal in learning an elaborated version of Runequest.
I just got a copy of RQ Glorantha. So far, it seems more like RQ2 with a lot more setting information. The clan and economic holding information reminds me of Pendragon - mostly in a good way. I got the rules it in a slip cover package along with the Bestiary and Game Master's Screen. I didn't think I'd get much out of the Bestiary, but it seems like it will actually be more useful than I expected. The maps and Sartar information in the GM pack seems like exactly what we wanted back in the early-mid 1980s, but did not have. I'm not far enough into reading the RQ rules to see how it compares with RQ 1-3. So far, it seems closer to RQ2 - which for me is a feature not a flaw and it seems to have mitigated at least some of the problems I had with Shamanism and Sorcery.
The RQ:G character creation is significantly more elaborate than RQ2 (again more like Pendragon) and it follows a family-oriented life path creation. It definitely presumes a more connected, homeland oriented style of play and some folks will like that. Those who prefer rootless, wandering heroes or who want to create a new character in < 10 minutes probably won't like it. The rules do include options to truncate or skip the background creation. If you don't want Glorantha or a setting that is like Glorantha with the serial numbers filed off, RQ:G may not be your best bet for system. I'd go with Classic Runequest 1 or 2 instead.
Until I've had a chance to finish reading and creating a few characters the jury is still out on what I think is the best system.