Space marine Scouts are some of my favorite models and units in the Space Marine Codex. I'm not sure what it is about these baby Marines, but they reek of cool factor. Go read Sons of Dorn or Courage and Honour and you'll see what I mean.
On the table, Scouts aren't as awesome as their literary counterparts, but they're still a fun unit to use. Every squad you field needs a sergeant, so let's dig into what makes a Scout Sergeant.
Scout Sergeants have the same stat line as every other Marine Sergeant, but have 4+ armor save instead of 3+ due to wearing carapace armor instead of power armor. Scout Sergeants can be armed with a shotgun, sniper rifle, bolter, or combat blade alongside their bolt pistol. Aside from the sniper rifle, the Scout Sergeant is the first unit leader who can take more than two "free" loadouts.
The shotgun versus combat blade/pistol debate is an ever-present conversation. It boils down to whether you prefer to have one extra die to roll in the Shooting phase before the charge, or one extra die in the Assault phase after the charge. Personally, I like the looks of the shotgun, so cool factor points win there. You can factor in Overwatch fire in the enemy turn as well, but depending on what you're going up against, you might be better served with the extra attack in melee over the extra shot in Overwatch.
Bolter-armed Scout Sergeants are also ok, and serve nicely as an objective camper or outflanking option. Sniper rifles now cost you a little extra, but it's not a crazy amount. The Scout Sergeant gains no true benefit by firing a sniper rifle, as all shots with Sniper are Precision Shots anyways. The extra 12" of range over the bolter is nice for backfield campers.
Scout Sergeants can get away with not taking any upgrades beyond a free swap and still be effective. The driving force behind Scout Squads are their special rules, not their armaments. Scout, Infiltrate, and Move Through Cover make them awesome disruption units and objective holders.
The Scout Sergeant can buy a teleport homer. It's an interesting option and a neat way to provide backup for your Scouts while also getting Terminators across the field without taking a Land Raider.
Meltabombs are also available to the Scout Sergeant, where they have a greater chance to be an offensive weapon than when placed in the hands of something like a Tactical Sergeant. The ability to infiltrate a Scout Squad and threaten enemy armor with the meltabombs is valuable. Put the squad in camo cloaks and infiltrate them into area terrain or ruins to help them stay alive, and then maneuver your way towards the target. Your opponent will have to expend some assets in removing the Scout Squad or risk losing their heavy armor.
Scout Sergeants can spend points to access any of the options in the Ranged Weapons and Melee Weapons lists.
Due to the flexible deployment options available to Scouts, all of the ranged weapons have some utility. The plasma and grav pistols are easier to get on target when you can Infiltrate and Scout, although they are still only a single shot per turn. However, if you're lucky enough to outflank on the proper side of the table, you might be able to get the proper angle for shots on a unit leader, or put a grav wound on a heavy firebase unit or something.
The combi bolter options are also useful in the hands of a Scout Sergeant. Instead of one plasma or grav shot, or slogging across the table with a combimelta or combiflamer, you can pop in closer by outflanking or infiltrating.
The combi option is the most solid selection for a Scout Sergeant if you plan to take some sort of upgrade over the basic bolter, shotgun, blade, or sniper rifle.
While Scout Sergeants do have access to all of the same melee options as other Marine sergeants, the days of melee glory are over for scouts. In times past, it was a viable option to take a powerfist and a combiflamer on a Scout Sergeant and outflank the unit to punch armor or drop the flame template on light infantry. In Sixth Edition, you can no longer assault on the turn you arrive from reserves or after making a Scout move. Melee options lose much of their utility under that restriction. The only melee upgrade I can really recommend is a simple power sword for "just in case" situations. The likelihood of a Scout Sergeant surviving to swing a powerfist or power axe isn't high enough to justify the cost.
The Veteran upgrade is almost always valuable for Scout Sergeants. The squads they lead are almost always operating deep behind enemy lines, or sitting behind your own lines providing token covering fire and solid objective retention. These situations requiring them to pass Morale tests to stick around or risk failing their mission entirely. The extra point of Leadership is important here.
The only Chapter Tactic that directly affects Scout Sergeants is the Black Templars. Rerolls to hit and rending in challenges can make a naked Scout Sergeant a little more useful, but not overwhelmingly so.