The Toughest, Hardest to hit, Sniper

[Trey Bright]

Chooses Tabaxi as race because why not. Also they give +2 to dex. That's good.
Then chooses....barbarian! Go primal path of the totem.
Take all of the eagle choices at ever totem choice for extra evasiveness and ability to shoot things a mile away from you in the dark.
Max out Dex and Con as soon as possible. If you're using standard point buy, you'll have one ASI left to take a feat after Dex and Con hit 20.
I would take mobile myself just for added maneuverability, but that's my personal choice.
For the sake of this character, his back ground is he was a slave to the drow, but managed to escape. He got to the surface and wound up near a settlement of high elves. He tried to ask them for help, but when they heard him say drow, they thought he was a spy or trying to set a trap and attacked him. He ran away.
He envies the birds. Wants to be able to soar like the wings of eagles. That's why eagle totem. He figures if he could just fly away from his problems, everything would be okay.
For this character, try and find the oathbow. Use it to kill everyone with pointy ears. Neither the elves below the ground or those above tried to help you in your time of need. They deserve to die. Take Elven and Undercommon as starting languages.
By level 20, because barb's get Con to AC, this character will end up with 22 AC completely naked. Add on some bracers of unarmored defense and maybe a ring of protection and possibly a cloak too if you want to go crazy.
You should be far back shooting enemies from far away, but if they do get in close, because of class and perk choices, opportunity attacks will have disadvantage against you. Enemies that you've attacked won't be allowed to attack you. With 50 move speed and the ability to fly whenever you rage, you'll be very maneuverable.

[JellyPooga]

Problem; at level 20, magic items aside, this character deals a mere 2d8+10 damage per round. +30 if you grab Sharpshooter and hit with it...and that's it. He's tough and maneuverable, but, well, he's simply not much of a threat because none of his class features really support his chosen role beyond being hard to kill.

Also, 22 AC isn't much to brag about at lvl.20. I hate to shoot you down (ba-dum tsh), but Rogue and Fighter do a much better of doing, essentially, the same thing and that's before thinkng about the specifics of the build or multiclassing.

[Unoriginal]

Isn't the idea behind sniping that you are hard to hit because you're far away and hiding?

[Trey Bright]

Problem; at level 20, magic items aside, this character deals a mere 2d8+10 damage per round. +30 if you grab Sharpshooter and hit with it...and that's it. He's tough and maneuverable, but, well, he's simply not much of a threat because none of his class features really support his chosen role beyond being hard to kill.

Also, 22 AC isn't much to brag about at lvl.20. I hate to shoot you down (ba-dum tsh), but Rogue and Fighter do a much better of doing, essentially, the same thing and that's before thinkng about the specifics of the build or multiclassing.

Eh, that's fair. I just liked the idea that you could have a character that has more AC naked then any other character. I suppose this is more of a fluff character then actually useful. Might make a better NPC...or just better to be played at lower level games you know won't be going in to higher levels.

Isn't the idea behind sniping that you are hard to hit because you're far away and hiding?

Yeah, but since that won't really stop wizards or other snipers, having a decent AC an HP pool are still important. I was just looking for a way to try and make a Dex Barb. I also liked the idea that since barbs add con to ac and get a con boost, they'll have the highest natural AC out of any of the classes.

[Greywander]

Maybe you're looking for something using an actual weapon, but I think the best long-range sniper is a warlock with eldritch blast.

Here's the build I'd recommend:

Barbarian 6, eagle totem for 1 mile vision
Warlock 9, for 5 invocations (agonizing blast, eldritch spear, repelling blast, grasp of hadar, lance of lethargy)
Sorcerer 3, for distant and quicken spell
Fighter 2, for action surge

Get spell sniper, and boost CHA to 20. Keep a dimension door in your back pocket for when an enemy gets close (which they won't unless they're very fast and very sturdy, or there are a lot of them). Eldritch blast base damage is 4d10 + 20, average 42. You can quicken and action surge to burst 126 average. If you just quicken, that's 84 average damage. So over three rounds (when you run out of sorcery points), you deal 292 average damage.

Honorable mention: Assassin rogue for autocrit on surprise round

If you want a weapon-based sniper, I'd go with a wood elf (for longbow, and easier hiding) rogue assassin with sharpshooter feat. Dip fighter 3 for action surge, go champion for improved critical. Deal 2*(4d8 + 10 + 18d6) damage = 182 average damage on a surprise round. On normal rounds, you'll deal 1d8 + 5 + 9d6 damage = 41 average, assuming you get your sneak attack bonus. During the first thee rounds of combat (compare to above) you deal 262 average damage.

Eldritch blast seems both more damage and more reliable. Plus, distant spell let's you snipe as far away as 1200 feet, and invocations let you slow, push, and pull enemies. Your damage is enough to bring down any adult dragon in 3 rounds, and nearly take down some of the weaker ancient dragons in the same time. With lance of lethargy and repelling blast, you can effectively slow their 80 foot fly speed to 30 feet, taking them down long before they get within range of you. Even if they do get in range, you can dimension door out of there and finish the job. Also, you can use a shield since spellcasting doesn't need two hands (unlike a bow). Your main weakness is tight spaces and enemies that resist or are immune to force damage.

[Lombra]

I believe that thieves make the best snipers: easy climbing and (almost) self-sufficient magic item use make them a good lone wolf scout, on top of the overall great rogue evasiveness.

[rollingForInit]

I'd that Hexblade blasters make seriously tough snipers. Eldritch Blast + Agonizing Blast for the actual sniping. Spell Sniper and/or Eldritch Spear for better distance.

With medium armor and shields the hexblade will have 15+2+2=19 AC (assuming 14 dex). D8's as hit dice is good enough. They get Shield as a spell, so they can always use that for emergencies. Maybe multiclass into a level of Sorcerer and grab Absorb Elements.

Not quite as outright tanky as the barbarian, but it with spells like Shield, Mirror Image, Misty Step, Greater Invisibility, and so on, it'll have a lot more defensive abilities to just not get hit, or to get out of trouble.

[Spacehamster]

12 arcane archer, 4 gloom stalker ranger, 4 assassin should be pretty efficient too, wood elf start stats 8/17/14/10/16/8, elven accuracy, mobile, sharpshooter, alert, skulker and +2 DEX from the 6 ASI’s.

[The Shadowdove]

One could also argue that the hardest archer to kill is the most clever and well prepared, with the most archery related augmentations and equipment. Mobility and endurance are merely bonuses to your arsenal when you are already a terrifying and unpredictable death machine.

[Naanomi]

Lots of good options... kind of depends on your concept of a 'sniper'. I've done the Eldritch Blast long range Nova; and the Wood Elf Assassin (with one level of Fighter) one-shot-one-kill stealth character

[Easy_Lee]

Kensei monks can use bows. Monks move fast, can disengage or dash as a bonus action for one ki, can bonus action stunning fist a target that gets too close and move away and attack it in the same turn, are immune to or able to break some conditions, and eventually gain proficiency with all saving throws. If being a highly evasive and hard to pin-down archer is your goal, I don't think you can beat this.

Tabaxi Kensei Monk with Mobile and Sharpshooter for ultimate evasion. Convince an ally to give you Haste and Longstrider and your movement speed increases to stupid.

For more damage, take just five levels and put the rest in Rogue.

That said, you're begging for your DM to surprise you. Maybe take that into consideration and go VHuman with the Alert feat instead.

[Relbin]

Valor Bard, Level 10, Longbow, Sharpshooter, Find Greater Steed (Pegasus). Shoot anything from 600 feet in the air while buffing yourself with swift Quiver or Improved Invisibility. Laugh while foes run for cover?

In smaller dungeons/inside, grab expertise in stealth and shoot while invisible or in the next room. Or wait for foes to stand next to a window. ;)