"Which New Vegas ending is the best" is a discussion as old as dirt here.
Now that New Vegas 2 might actually be happening though, the question arises again (because I don't think they're going to create four vastly different stories to accommodate the various endings of NV, they're just going to pick one and make it canon I'd say.)

So, which ending would be the best, not to live in personally, but to use as a springboard for a sequel?

I'd say it's a House victory, with an independent Vegas from NCR. The Khans and Followers go north to form a super-empire together, combining their individual powers. Caesar dies and the Legate retreats the army to fight another day. His finall dialogues portrayed him as someone who wasn't the mindless brute I'd expected. In fact, I think he's better than Caesar (who I saw as a pseudo-intellectual blowhard, blinded by his own rhetoric, with no long term post death plans.)
The NCR's failure in the mojave causes a civil war or rebellion or other conflict between the various classic towns/areas in California.

I think a legion victory would suck for a sequel because they left ruins in their path, killing almost everyone in their ending. Vegas would end up a dark ruin filled with idiotic anti-drug raiders.

The NCR, being prudish and having made illegal so many vices, would ruin what was special about the NCR so I would hope they're not in charge of the Mojave in the sequel.

A courier victory? I think Mr. House is simply more interesting, what with his plans for space travel. But... his voice actor has passed away.

Personally, I'd like to see the NCR ending be used, but that's just because I like their Wild-West style, cowboys, and taxes.

NCR would be my gut reaction but I think I'd rather see something that's a mix of NCR and House. No Yes Man or Legion endings.

I think I'd like to have House around as an ongoing background player. It would be a shame to kill him off as he makes for a good/interesting power dynamic between the NCR government and Vegas itself. There certainly would be friction but they both kind of need and tolerate each other.

If...IF...New Vegas 2 ever happens they won't just pick one of the game's endings. There will be significant details added and left out. Don't expect to be given full details and firm answers on what happened to Courier Six.

I personally think that if Fallout New Vegas 2 ends up being more than a dream, they'll focus on something along the lines of making whatever choices happened in the past more ambiguous. Given the design philosophy of Fallout New Vegas, it would make even more sense if Fallout New Vegas 2 took place in a world where the courier didn't even exist and the Mojave became a massive bloodbath for years.

If they do choose an ending, I think they'd go with NCR. Things probably wouldn't be happy and their territory probably not secure. Maybe something like an NCR civil war pulling troops away and several groups have risen up all vying for control of the Mojave territories.