AMR. By far an AMR. You get one of those with AP ammo, aim it well(Aim for the head), do it from a sneak, and they drop in one shot.

Bottlecap mines are much less common in NV and Deathclaws are much tougher

I don't have any sniper rifles so im stuck using the cowboy repeater or hunting rifle and explosives,
can someone tell me where I can get the anti-material rifle, gauss rifle, or other powerful rifles?

Wikia.

Gunrunners by the freeside east gate has the Anti material rifle, sniper rifle, trail carbine, marksman carbine and other nice gun toys. I bought a gauss rifle from the Brotherhood of steel merchant.

boxing gloves are good for one or 2 ,knock them unconcious then change weapon and kill them whilst asleep...this is best when only 1 is encountered

Alien Blaster will one shot anything in the game, even Deathclaws

I'm answering straightforwardly: Use a Fat Man.
You did not say which one is the most reasonable as in not wasting ammo, you just asked for the "best" weapon to take one out, as in the most effective, and it is the one and only (MIRV is an exception) FAT MAN!

Alien Blaster

Nuff said

In addition to what I've said, I will give you Deathclaw "tips" for example:
1. The "tip" of their claws are really sharp, they could even kill you!
2. The "tip" of their horns are sharp too, although they don't use the horns very often, it is very sharp
3. The "tip" of their teeth are also sharp, the teeth are used to chop, grind, and churn the toughest bloatfly (or human) meat out there!
There you have it, Deathclaw "tips"!