+1 sword is actually your best use of myth if you're doing Coil as anything else drops there before turn 5. Ring is the same deal since you want one myth ring + one allagan ring ideally. Rest is just a matter of figuring out what's best in slot for you and spending your tomes accordingly.

Alternatively if you don't care about Coil gearing then just wait 'til 2.1 where the real AF2 will come out and you'll have to grind even more tomes for it but it'll be superior to AF+1/Allagan gear until later turns etc. But no one wants to do that apparently, so I'd recommend +1 sword first. Then get 75 next week and get your ring. Then decide from there. If you're not running Coil then just upgrade w/e.

Edit: Oh, the head also drops on turn 5 (apparently) so you can spend your myth on that too but it won't be as much of an improvement compared to the ring.