Well you can mod most guns to be either semi or auto. I think the only exception are sniper rifles, non-laser shotguns and non-laser pistols.
That said, I think Rifleman is by far the strongest of the four choices we get. I got to about lvl 20 in my previous playthrough and used rifleman from the start. Early game was fairly comfortable because the perk also covers shotguns which are devastating when you're starting out. Having good damage on sniper rifles is also a huge advantage early in the game.
On my second character which is now lvl 55, I've played a crit luck pistol build for the first 50 levels. Early game was a pain in the ass at times and the build only fully developed once I could chain crits all the time, which was at lvl 40+. It also forced me into VATS all the time which was the #1 reason why I respecced (playing on the PC so was I able to use a tool for it). The problem with pistols is that the only really damaging caliber is .44 and the ammo for it is pretty rare, even with a few points in scrounger perk. So instead of that I used a modded Righteous Authority with a pistol stock which worked fairly well and the ammo for it is plentiful.
At lvl 52 I respecced using the
Fallout 4 Respec Tool and decided to go for a non-vats Commando build. Damage was ok but I felt like I was wasting too many bullets. After about an hour my supply of 4500 .45 caliber bullets decreased by 500, and that was with an Overseer's Guardian rifle (automatic receiver). With any other rifle I would've spent way more bullets. I barely noticed the stagger from Commando perk, the targets either died so fast that I didn't notice or it just didn't seem to proc.
After this short test with full auto weapons I've decided to play it safe and fell back to Rifleman. It basically confirmed at high level what I've known all along - it's pretty much perfect for the way the game is currently balanced. It's good for ammo conservation (and covers a wide selection of different calibers) and it's got a builtin armor penetration mechanic which plays really well into the way armor works in this game (heavy penalty to high rate of fire low damage weapons). It also covers long (sniper), mid (assault rifle) and close (shotgun) range weapons, making it by far the most versatile perk.