Depends on your definition of "best".
By mine, they're far from the best rifle ever made.
The bolt handle's too short & inefficient, the handguard's prone to warping because it's too thin, the design is difficult to mount optics on, the iron sights are typically poorly regulated, the build is relatively un-refined, the triggers are anything but crisp, and the safety is quite awkward to use.
They are usually reliable and robust, but that doesn't make them the best, for me. They worked, but other designs worked better overall.
I have three currently, have owned five over a period of several years. One I have now has over $700 invested in sporterizing, which is still about twice what it's actually worth.
Not saying you, or anybody else, should not buy one, just disputing that "best" label.
Inspect carefully before you buy. Check the bore, look for pitting on any exposed metalwork (and hope you find no rust craters under the woodline), check the front end of the handguard & stock to make sure they're not warped and putting pressure on one side of the barrel or the other.
Check for a loose handguard and/or barrel bands & retaining springs.
Make sure the barreled action doesn't move back & forth in the wood.
Learn how to totally disassemble the bolt & remove all cosmoline. Make very sure you get every last bit of cosmoline out of the chamber & bore, or you'll run into the famed Mosin Sticky Bolt Syndrome.
Not all parts are serialed.
Models will be some version of the 91/30, with the most common having round receivers, lesser numbers of the older pre-1930s with hex receivers.
Tula rifles are generally made better, rifles after about 1943 may show a decline in machining quality.
The sights will probably be off & shoot low. That can be remedied by heatshrink tubing on the front post or by buying an aftermarket front sight assembly.
They were zeroed and carried with a bayonet mounted, but you don't have to shoot it that way.
Denis