I've GM'd Rise of the Runelords for our local group. It's good enough of a story that I wouldn't mind going through it again as a player, though.
I'm not sure I'm a fan of the Mythic rules. I agree that combat in general sort of falls apart at high level.
Mummies Mask might be interesting. Not my usual favorite genre but for that very reason might be fun to explore.
Iron Gods: I think if I'm going to play in a tech RPG, I'd rather play one in a campaign setting based on it instead of trying to shoehorn it into Golarian.
Giantslayer: I'd be fine with this.
Strange Aeons: I haven't really looked directly at the AP at all yet, but I've browsed through the Horror Adventures book and honestly feel like the expanded rules for fear and corruption could add a lot of flavor. My first thought was actually that I'd love to play a Carrion Crown game with these modifications.
I've played well into the second book of Carrion Crown in PbP and absolutely loved it, but we had to invent a lot of the "horror" feel ourselves because the mechanics don't support it all that well (and certain classes that are immune to fear entirely). I feel like those additional rules would only make it easier to get into the intended flavor of the AP. That PbP was still going strong and I actually was the reason it stopped, due to me withdrawing from all but one of my PbP games towards the end of my marriage in an attempt to focus more on personal life issues.
I know you don't have the AP, but I'd be willing to chip in to get you a copy, just because I'm so excited about the idea of running it with Horror Adventures rules.
If not, my top votes would probably tied for MM, RotR, and Giantslayer.