Although you're not wrong, I don't think you're right either. Yes, a guild that has lasted longer has had a longer time to make an impact on the realm and make a name for themselves, build a reputation and community and whatnot. However, this is not mandatory. Having a level 25 guild is not really relevant in the sense of PvP because, well, it's just irrelevant. For the sake of argument, let's say a fully active guild such as... FLAWLESS. Let's say they decided to disband their guild and remake a new guild, with the same people, but a new name and a lvl 1 guild. Would that make a difference? No, no it wouldn't.
Also, if a guild has had a long time running with an active roster, it does not make them the best of the moment because of their accomplishments from the past. Jinxed is for instance such a guild. They were highly active and succesful at the early months of the realm, but have recently faded into inactivity. Are they still "the best"? No, no they are not.
<D O N O R S> are definitely a new guild, and they are very small in numbers (I think they're <30 in unique members) but in overall, the way PvP is active on Neltharion makes them very viable to be considered the best - currently. A quantity of members means very little in arenas and battlegrounds, the only value a large amount of members has is in world PvP such as capital city raiding. But then again, there's really no competition with other Horde PvP guilds in that regard, so I wouldn't say that matters.
PS. I consider every PvPer from Alpha & Omega members by affiliation of D O N O R S. A guild in the regard of PvE is a unity of schedule and communication, but in PvP it doesn't hold the same necessity. A PvP guild is more like a brandname, and in that regard the name D O N O R S bring the most to the PvP scene on Neltharion from Horde at this time.
Edit: Also;
No, I'm not basing their value on the fact that they're donors, I'm basing that from the performance I have seen from said individual players. Are you trying to negate their value as players due to the fact that they're donors? Because that is donor bashing, and a very irrelevant argument to even make in this discussion.