I would go in this order:
Avalon>Valor>Assault>Ares
Why are you placing Assault breastplate before Ares's Cuirass here, Tymon?
I assume you mean in which order you'd prioritise use, and in which order to obtain? In which case, surely Ares's Cuirass is first.
Breakdown of the benefits of
Avalon breastplate:
1. +3 Enmity can be picked up from Adaman Cuirass,
2. -5% MDT can be practically attained with Coral Scale mail (+1),
3. And the acc can be beaten by Haubergeon rather easily.
With macros and the above pieces, you can get everything Avalon Breastplate has for about 2,000,000 G less. (With the added bonus of not looking mysteriously like a lobster.)
Ares's Cuirass provides the best option for idling and kiting due to it's refresh and HP/MP/DEF (with the exception of Valhalla breastplate if you're not meleeing with Atonement, or macroed for utsusemi down/phsyical WS/Warrior or RDM sub etc). You could say there's V.Cloak for refresh, but the HP/DEF loss is quite significant. On top of that, Ares's Cuirass provides a decent idle piece regardless of whether shadows are up or not -- V.Cloak doesn't. Ares's Cuirass provides the defensive capabilities of M.Body AND the refresh of V.Cloak. (With the added bonus of looking rather badass.)
Why is it lol? That's some massive elemental resist right there, for those nasty "Throw the Paladin at Sarameya" fights.
I think it's rather silly to pay 2M for 10 extra fire resistence, personally (you can already pickup +5 Fire resistence for around 10,000G).
99% of the time you're not going to have a complete resist set. Be that due to a BRD who doesn't have the best instrument for carol or capped/meritted singing/wind/string, or a WHM who doesn't have capped bar-spell merits, blessed briault (let alone capped Shellra V), capped enhancing, or just your own gearset that may be lacking complete resistence. Either way, it's going to be incredibly rare that you would ever be "missing" that 10 resistence more. My point is, unless you have 100% of all the above things to maximise your elemental resistence, it's not worth the huge pricetag.
With the exception of Sarameya (maybe), you won't notice a slight bit of difference with 10 resistence. Even for Sara, I can't imagine how 10 resistence would be anywhere near essential or necessary. And besides, you'll need a mix of MDT, Elemental resist and MDB. Of which, one of the very best MDT pieces is on the body only. Coral scale mail / Avalon brings ~5% MDT, which is incredible (unless you have Aegis, in which case you'll be capped with Shellra V 5/5).
Hope that answers your question Trist.