Kilgore Trout
Gyro Captain
Realistically, I think whoever can secure the support of the Boomers, owns the Mojave. Between Air power and the sheer amount of ordinance they're packing, they can just rain flaming death on any of the other factions.
Numerically speaking, though, Boomers have a lot of the same problems as the BoS. They're also even more cripplingly xenophobic, to the point where they won't even leave Nellis.
There's plenty in the wasteland that's capable of knocking a WW2-era bomber out of the sky, especially given the scarcity of spare parts and the Boomers' apparent taste for extreme low-altitude bombing runs.
Short of a Vertibird (which are extremely rare) or a missile launcher, I'm not sure what could manage this, even at low altitude. Are you thinking a shot from an AMR or Gauss Rifle? Admittedly I'm not a ballistics expert so maybe I'm overlooking something obvious.
The Boomers don't really have enough firepower to maintain a full scale attack or war, their plane is made up of scrap so I am assuming they have to maintain it after every flying session and they don't have the numbers to go to a full blown war. I mean that was their reason for holing up in Nellis. They are only good allies during that Skirmish in Hoover Dam, where the Legion and the NCR are busy with each other to properly react to the Boomers assault.
If we're only talking about hoover dam, I'd say the boomers win hands down.
Long term? I think it's either the securitrons or BOS.