Moltres Fire ou Legacy

I personally prefer Sky Attack to any of the 1 bar fire moves so if it were me I'd keep it. Moltres is almost impossible to dodge with and getting off 2 Overheats is a feat unto itself.

I would also keep sky attack. It shares effectiveness with fire on grass and bug matchups and makes it a nice pokemon to use vs fighting. I am really happy with my powered sky attack moltres in machamp solos.

Keep it. It has shared effectiveness and is useful for many raids. Two bars are generally better than one, so you can use it more often.

Also, there are other options for fire attackers. Entei is available in research, and we get Heatran next gen.

I would also suggest keeping sky attack. As I mentioned in another similar thread, SA is legacy for Moltres, so you can't get it back should you TM away from it. It's outperforms OH against Grass/Bug types, so it will always have a niche there. Plus, at some point, we'll get Heatran in Gen 4 who will likely threaten other Mons with double Fire-type movesets. SA may be the move that keeps Moltres relevant after Heatran's release.

Parroting what has been said already, keep Sky Attack. Looking at gen 4 predictions, Heatran needs a new moveset that's better than FS/OH, an already stellar moveset, to outclass SA Moltres against grass and bug. SA Moltres is thus a much safer investment at the moment than double fire.