That depends on your style, honestly. Being a musketeer, her hit points are low and her damage is lower than swashbucklers, buccaneers, and privateers. There are two ways to go:
1) Increase her average damage to the enemy. This is done by putting in as much agile and accurate as you can, followed by rough. If you have anything left after that, I'd put it in dodge.
2) Increase her ability to survive the fight. This means putting it in dodge, armor, and tough. Anything left after that, I would put in resist.
Now, on to the epics: I have always liked overwatch, as it makes her fire when someone gets in range. Secondly, I like quick adjust as it gives her a second chance should she miss. She already has Double Tap, so, I would increase that last. Quick draw is nice, when someone is about to fire at her, it lets her get in the first shot. Return Fire lets her fire back if someone fires at her and misses. Similar is True Grit which lets her fire back if she DOES get hit. In general, I prefer epics that will set her off regardless of their class and on approach, rather than when they are going away (Parting Shot).