This question is more complicated than it appears. Please remember that DPS is largely meaningless in this genre, and not only because mitigation is much spikier than something like an MMO.
If the real question is, "what melee speed is best," well, it depends. For obvious reasons, you want the biggest damage weapon regardless of speed for a sneak attack. For fighting another melee weapon wielder, you almost certainly want a faster weapon than they have, to get as much attacking as possible in between blocking their attacks, and their blocks will mean less per hit.
For everyone else, well, that depends on their armor. The breakpoint for 50% damage reduction is 1:1 damage to DR. For ratios less than that, it ramps sharply up to 50%, and for more than that it slows significantly. (1:20 is ~83% reduction) The upside is, if the average enemy has less DR than your slow weapons, but more or equal than your medium weapons, your slow weapons will do significantly more net damage. If enemy DR is equal to or greater than your slow weapons, it doesn't really matter what you use.
The point is, there's no hard or fast rule to determine melee dps. I wish I could just give you a coefficient for each speed to divide damage by, but it just doesn't work that way.
Plasma weapons in Fallout 4 are relatively rare because of how powerful they are. You'll almost certainly not find one before level 20 and if you do you'll most likely be out of ammo for it with no way of getting more as the plasma cartridges are some of the most expensive ammunition in the game. Ultimately random drops are random and so you may find them anywhere, higher level enemies are far more likely to carry one than lower level enemies though.
You're far more likely to find a plasma pistol than any other form factor, but this can be upgraded into a rifle using the weapons bench as required, and to that end, there is normally a plasma pistol being stored inside the Mass Pike Tunnel West (near Diamond City), inside the room containing a steamer trunk with a Plasma Pistol inside.
The "best" plasma weapons in the game are the unique faction variants such as Experiment 18-A, which can be purchased from a synth vendor in the Institute, and the AX90 Fury or Sentinel's Plasmacaster, which can be purchased aboard the Prydwen.
Best Answer
If you're aiming for the highest damage per bullet, you should have a look at the Railway Rifle. It starts out with considerably higher damage unmodded in comparison to the Hunting Rifle.
However, you might feel it has a very "un-snipery" feel to it because it does not look in the slightest like a weapon made for sniping in the usual sense. In that case, yes, the Hunting Rifle is your best bet. I had no raw one at hand, so here are the fully modded stats of a Lucky Hunting Rifle with all damage and sniping components:
I would look thoroughly for a rifle with a good legendary mod though, seeing as 139 damage is pretty meagre in higher difficulties. Aim for one of these: (somewhat in order of expected payoff)
EDIT:
Ammunition did not pose a problem for me. As you can see on the screencap, I have ~1200 shots. Another 1500 are on Nick because I gave him a similar weapon. A clip for the best magazine mod holds 10 bullets if I'm not mistaken, with which, when looking at the Fire Rate, it generally takes FOREVER to deplete 1000+ rounds.
Also, if you pick up the Scrounger perk (additional ammo found throughout the world), you will find boat loads of .50 ammo. This stuff is everywhere and I have absolutely no use for it. Maybe it's just a troll and you'll find plasma cells when you're using that Hunting Rifle, who knows...