Ranger is in a great place right now. As an owner of multiple RNG RMEA weapons, I'll give you a brief rundown of what I like about each, and what I use them for.
*warning* I'm much more aggressive, sometimes to a fault, than most rangers out there. Oh well. This job functions quite well when weakened^^
1. Fomalhaut- My main weapon of choice for any general purpose fight. Incredible damage on the gun, highest damage on the accompanying ammunition. Last Stand is our bread and butter these days, and scales with TP, and the Foma's TP bonus only feeds the beast. In addition, its a great weapon for Trueflight, meaning you're covered for both physical and magical weaponskills.
2. Gastraphetes- My favorite weapon. This thing is a beast when afterglowed- boosted damage to Trueflight, AM3 giving ODT on regular shots, and the most complete ammunition in the game- Quelling Bolts. The Gastra itself has Barrage accuracy trait on it, and the bolts have "Barrage +1" to boot. Its purely a weapon for when you're using Trueflight....I'd rank it 3rd for Last Stand.
3. Annihilator-3 things I love about my Anni....first, a STRONG Darkness skillchain option over Wildfire. Second, the aftermath WILL proc on Barrage, as opposed to the mythic, which will not. And lastly, the wonderful hate mechanics of Coronach, as well as the additional "emnity -25" on the aftermath(not to mention a 10% ratk boost with the AM). Its not going to win any parses against equally geared Fomalhaut RNGs, but they might live longer in a drawn-out fight.
4. Fail-Not- these days I only use this weapon to get the Chrono Arrows from it. Its a great bow, but bows just are lacking compared to marksmanship for the most part.
5. Gandiva- my newest addition, and I'm still getting used to using it versus a more straightforward weapon like the Foma. But it is absolutely incredible if you put the effort into it. The ODT on the aftermath is so damn potent, and Jishnu's Radiance definitely packs a better punch with the +50 DEX on the bow. But I don't use the Artemis Arrows with it- just Chrono Arrows and it makes a BIG difference.
---Now for the two I don't have--
6. Yoichi- its great attribute is the ranged accuracy, something we just don't need as much of these days. Its underpowered compared to other bow options. I never reccomend doing this RMEA.
7. Armageddon- a GREAT all around weapon, that gives you the flexibility of both physical and magical weaponskills, possesses +50AGI which is just godly for a RNG, and packs the ODT aftermath the same as the Gandiva. Its not going to be the physical weaponskill beast that Foma is, or the magical monster that Gastra is, but its a great compromise and also have some uniqueness to the TP phase. Plus, it can pull double duty for a COR. Again though, it benefits from stronger ammo (Like Chrono bullets from the Fomalhaut), but if you can get the ammo, many times you'd be better off just going Foma over Arma.
So wrapping up- If you CAN get aeonics, go Fomalhaut. If you can't, Annihilator or Armageddon are going to be great all around options. Then, once the bug has bitten, start getting specific and build up that gun rack!