Continuing my series of small, themed Space Marine armies, this time I'll go with the White Scars, who arguably have the best of all the chapter tactics.
The strength and theme of the army is going to be bikes on bikes. They are the benefactors of the Chapter Tactics, and are already arguably very strong units. With Captains on bikes unlocking Bike units of 5+ as troops, this list is going to practically build itself.
The first place to start is obviously a captain on a bike. To boost his kit a little we'll give him a combi-weapon (probably grav, but plasma might not be a bad idea) and a power weapon. He's going to get in combat at some point and we might as well make him capable of doing some damage before he and his unit Hit and Run.
Next up is troops. With the captain on a bike, units of 5+ bikes can be taken as troops. I'll take 4. The first two squads I'll max out, giving the attack bike a multi-melta for some anti-armor and the rest of the squad either two plasma or two grav guns. Being relentless, they don't suffer from the grav gun being a salvo weapon. Combat squading will allow these guys to let the attack bike hunt tanks while the 2 special weapon bikes and sergeant with combi-weapon bike go hunt elite infantry and the like.
The remaining squads will get bumped to five and add another attack bike in one. Giving the attack bike squad meltaguns and a multimelta to be a dedicated anti-armor unit. The final bike squad will match the combat-squad halves of the two big squads with two grav or plasma guns and a combi-weapon on the sergeant.
That's really it. While the bikes are fast and tough, they are expensive. This is a pretty one dimensional army, but definitely has potential and could cause problems. Not to mention, every unit will be scoring, which is nice.
Captain: Bike, Power Weapon, Combi-weapon (135pts)
8 Bikes: Attack Bike with Multi-melta, 2 Grav-guns, sergeant with combi-grav (263 pts)
8 Bikes: Attack Bike with Multi-melta, 2 Plasma guns, sergeant with combi-plasma (263 pts)
5 Bikes: Attack Bike with Multi-melta, 2 Meltaguns, sergeant with combi-melta (190 pts)
5 Bikes: 2 Grav-guns, sergeant with combi-grav (145 pts)
Despite the small size, this army is definitely tough and packs a punch. It's also fast and while small in number, it's not that small with 30 models in total, compared to 36 models for the Raven Guard and 28 for the Salamanders. This army, I would bet, would also be a good basis of a competitive tournament list. Dropping the two smaller bike squads would also get you a good allied detachment for another 1k list that would complement it well.