Which weapons are considered to be the best? Do you have to invest in legendary weapons to keep pace with players that have them?
Legendary variants are just skins. They have no statistical advantage over the base models.
Most guns are, more or less, useful in the game depending on your play style. Rather than trying to find the best guns you're better off equipping a loadout that most appropriately accommodates your own strengths and weaknesses. What you want to look at with weapons is;
a) What type of ammunition do they use, and their damage/effective range. Damage is obvious, effective range far the bullet can travel and still reliably kill with a headshot. Lesser ammunition types generally have lesser effective range.
b) Rate of fire. Powerful weapons usually shoot slower, and many rifles are single round reloads.
c) Reload speed. Tied in with the above. Because every shot in HUNT counts, a weapon that holds a lot of ammo and shoots quickly might have a poor reload speed. Whereas a single shot rifle might have a lower rate of fire, but a more consistent and manageable reload speed.
d) Muzzle velocity. Weapons have no bullet drop in HUNT, but they do have ballistic travel time. If you're planning to do a lot of long range precision shooting choosing high ballistic, fast muzzle velocity is probably beneficial.
But the ins and outs of HUNT are that no super powered gun is going to save you over other players that can still kill you one shot to the head with the starter pistol. On the higher, more expensive and all round reliable end I very much like the Lebel 1886 and Mosin Nagant. They're your classic bolt loading rifles that do a lot of damage, have high muzzle velocity, scoped variants if you need them, and are wonderfully accurate. Downside being that they use rifle rounds, which are less common on the map and supply less rounds from ammo crates, and also have slightly obtuse reloading. If you try to reload when there are already rounds in the chamber (eg: half a magazine), you'll lose the round already chambered as you go to put in more. You need the Bulletgrubber perk to offset this. Meanwhile the Martini Henri is a single shot rifle using the same ammunition type. Obviously much more disadvantaged for that reason, as you need to reload after every round. It also has lesser muzzle velocity, so over distance you need to lead the shots more. But the reload is pretty snappy, you wont accidentally loose rounds, it has the same effective range
and technically does more damage. And it's about a 1/3rd of the price.
The Caldwell Pax is probably the best all rounder side arm. Not fast firing and it does slower reload, but it hits hard, shoots straight, and uses medium ammunition which is common enough. It's just a solid revolver to pot shot at targets.
But yeah, you more need to think about how you're going to play. HUNT is the kind of game where in the right context you can clear up players with the most expensive guns in the game just by using an axe, because melee is so fucking dangerous. The Nagant M1895 is the cheapest starter pistol in the game, using standard pistol ammo. But at the right range a shot to the head will instantly kill anyone, and its decent firing rate makes it still useful to chip damage.
The only major weapon factors to maybe avoid or be considerate of are;
a) Crossbows are deadly but hard to use due the low range and quick drop. They also have goofy sights. Better off practising a lot first.
b) Spectre 1882 Compact has fucking atrocious spread, the worst of all the shotguns, and is probably the one gun I'd avoid. The pump action sounds appealing but it's actually quite slow, and given you have to use it in close quarters you're better off going with another shotgun.
c) Avtomat and Nitro Express Rifle are monster weapons with their own flaws. Both can tear opponents apart in the right context. But the former is forced burst fire and eats up an ammunition absurdly quickly and has shit handling. The latter blows things to smithereens with absurd damage, but uses the rarest special rounds and has an awful sight. Both cost a fucking fortune.
That's about it. Sorry for the rant, but yeah. I think getting into a mindset of "what's the best gun" will inevitably lead to more frustration as you'll get killed by lesser guns and feel robbed. Some guns are definitely weighted as most advantageous in the grand scheme of things (Lebel and Mosin Nagant), but they still all have their flaws and situational context matters most. HUNT plays best when you're not getting in reckless shootouts, but being very careful with your positioning, pathing, noise, deception, trapping, etc. You can be the best rifle shot on earth using a scoped Mosin Negant, but if you're not careful you're still gonna get crossbowed to the head from a bush, or flashbanged and axed while defending the boss.