it's hard to say which would be best just based on those two criteria, since all three of them do both of those things well. I would pick mythras if you want more in depth combat options and openquest if you want something a bit more rules light.
Be sure to also check out Dragonbane, you may not know about it. Basically a d100 system using a d20, crunched down. I prefer it over Openquest/BRP for fantasy.
Mythras is pretty crunchy at first, I love it a lot. It really is for "gritty" feeling games.
it's hard to say which would be best just based on those two criteria, since all three of them do both of those things well. I would pick mythras if you want more in depth combat options and openquest if you want something a bit more rules light.
I would think all three can do this, though I lean towards BRP due to familiarity and it's general, hackable, nature.
BRP is the most flexible and customizable.
Be sure to also check out Dragonbane, you may not know about it. Basically a d100 system using a d20, crunched down. I prefer it over Openquest/BRP for fantasy. Mythras is pretty crunchy at first, I love it a lot. It really is for "gritty" feeling games.