The Bull.
Why?
In my opinion, if you do your best to act neutral during the Second Battle of Hoover Dam, the Bull, to me, comes out on top. On one hand they simply show superior tactics in that they acknowledge their weakness of close-range, and thus they stay within doorways and pop out at people as they approach doorways; the NCR just says "herp derp defend dam" with little to no tactics. On the other hand the Bull seems to lose the top of the dam but win the inside; I'd say that's more important, imo.
If you disagree with that, let me also point out this: do an NCR playthrough, do a Legion one. I've done both, and what impression did I get? That the NCR is caught with it's pants down and YOU have to play hero for them, whereas for the Legion, they're organized as hell. I literally don't think I had to fire a shot for the Legion until I got to the part that conveniently puts you alone. But yeah, DEFINITELY felt like the Legion had it better organized and easier.
So let's assume the Bull wins the Dam and Vegas. What's next? California. California is far too big and spread too thin, and the Bull is incredibly brutal, willing to do anything to win. They'd "cut the supply lines" and constantly wage war on settlements that weren't properly defended, and given how thin the NCR is spread, I'd imagine there's a lot of things. It's just a nightmare setup for the NCR; their biggest weakness would be exploited by a group that exploits this exact weakness regularly in wars they wage. At least with Vegas, that was the front line, with all NCR units funneled into one general area. The moment the Legion establishes a good base in Cali though, the NCR is in trouble.
You do realize that you can hardly intervene at all in the Inde ending, and the NCR still defeats the Legion at Hoover Dam, the only thing really differing from their ending being that they get stabbed in the back afterwards. Fortifying well-paced strategic strongpoints in an otherwise empty and featureless desert is hardly a poor strategic decision, but of course, all you care about is 'tactics.' Never mind that lying in wait inside a doorway is a horrible tactical idea, as doors are natural chokepoints (and kill zones). I would hate to be under your command, because awful decisions like that can get an entire squad wiped out by a handful of shooters. I struggle to think of any evidence suggesting they 'win' in the lower levels of the dam short of Courier intervention in their ending, because, as I believe I had to state earlier,
a small degree of tactical surprise does not offset the high state of operational and strategic readiness amongst the garrison at Hoover Dam. They may not have predicted them to come from the intake tunnels per se, but they had patrols on the lower levels to defend themselves against Legion sabotage and infiltration. Which is what happened, and they engage Legion forces coming in the lower levels rather promptly. So much for their brilliant 'tactics', eh? The overall operational readiness of the Hoover Dam garrison meant that despite the Legion's tactical svcker punch, the NCR was able to quickly respond to these Legion attacks.
Unfortunately, If I can recall, the only time we really go on the lower levels is on the NCR and Legion endings, and they win in their own respective endings by default, so there's no definitive answer as to the outcome of the battles in the lower levels, but since the NCR reaches the Legate's camp in the Inde and House endings and Oliver gives pretty much the same speech as in the NCR one about how satisfying it is to see the Legion rout, It dosen't look good for them.
Btw, I had a much different impression during my playthroughs. It just so happens I didn't feel like I had to play the hero in order for the NCR to win, and the NPCs around me more or less cleaned house on the NCR ending, particularly in the turbine area at the very bottom level where all the rangers spawn. The only reason I had to fight at all is because the Legion troops don't spawn until you get close enough on the map. The only thing that I saw as indicating a 'lack of prepardness' was the occasional line of frantic pvssyr over the Emergency Radio broadcast (although one of them turned out to be an 'Aliens' reference), but then again, the opening moments of any major battle is always chaotic, and it seems unreasonable to conclude that they were disorganized.
And in regards to your last paragraph, I would suggest you look up the concept of "Interior LInes". The NCR's main problem in the Mojave is its distance (and that distance complicates their logistics), and the Legion's relative proximity to the theatre of battle (meaning their supply lines are much shorter). If the Legion were to take the Mojave, and that's a big, big *if* right there, as there is little historical precedent for a preindustrial military conducting a successful conventional invasion of an industrial power, their roles would be reversed, as the Legion struggles to maintain its increasingly long, vulnerable supply lines in territory with hostile populations while the NCR continues to enjoy all the advantages a defender has, particularly when said defender has a massive advantage in firepower. A Legion invasion of the NCR would only get harder, not easier, the farther they get.
Frankly, in the short-term, neither side has shown the capability to bring about a decisive and total victory, with the NCR currently facing steep, but transitory problems at home (the BoS war and the resulting economic damage) while the Legion is hardly equipped to invade a much more modern country well in their own turf due to systemic problems rooted deep within their country (their relatively primative state.) If tribal, preindustrial armies, even ones with (some) modern firearms could have conquered the industrial West, they would have, and today North America today would be carved up into several separate nations by the various native american tribes. But that's not how history turned out, did it?