For the most part, Chapter Tactics are very similar to the previous Codex. Like Warlord Traits, there's just small tweaks.
Ultramarines: The Ultramarines Tactics are still the same; they can use each of the Combat Doctrines once per game. However, the Doctrines have changed a bit. Now they're much more symmetrical and easy to understand. Each of them can be used at the beginning of the Space Marines' turn, and last a game turn.
Tactical: All affected units re-roll 1s to hit in Shooting and Assault, and Tactical squads re-roll all missed to hit rolls in both phases.
Assault: All affected units re-roll 1s to hit in Assault, and Assault squads, Bike squads, Attack Bike squads, and Centurion Assault squads re-roll all misses in assault.
Devastator: All affected units re-roll 1s to hit in Shooting, and Devastators and Devastator Centurions re-roll all of their misses in Shooting.
On first pass, it looks like the Tactical Doctrine is the strongest. After all, it affects both Shooting and Assault. However, you have to look at the units that get the full benefit. You will likely have more Tactical squads in your army than any other type. However, Tacticals won't be carrying your most dangerous weapons. Their re-rolls will mostly be on bolters, while the Devastator doctrine may allow re-rolls on lascannons or grav cannons. The choice of Doctrine each turn is not an easy one, and the decision between Tactical and Devastator will prove very important. Assault will likely be more obvious, as there are generally very few turns in the game where you'll be making a pivotal assault.
White Scars: Born in the Saddle has been re-worded but stayed the same, granting Skilled Rider and +1 S on Hammer of Wrath hits. Fight on the Move has changed a bit. All models now have Hit and Run, including Terminators and Centurions. Also, all models can re-roll the dice for Run moves. I like this little addition, because it gives something to White Scars that are on foot. I for one would love to see some White Scars Tactical Marines on the table.
Overall, the White Scars got even stronger. I do think that they encourage a bit more diversity in army construction, as White Scar infantry will be a bit faster than standard. Still, Bikes will be the strongest builds.
Imperial Fists: Bolter Drill works the same, but now affects Sternguards' special ammunition. Siege Masters still gives Devastators and Devastator Centurions Tank Hunters, but now gives all units re-rolls to damage buildings and +1 on the Building Damage Table. If we ever see more buildings on the table, this will be big, but at the moment, it's just a very flavorful addition that will make for some awesomely cinematic, but rare moments, when your Tactical squad blasts its way into a Fortress of Redemption with melta-bombs.
Black Templars: Templars still have Crusader and Adamantium Will, and still can't take Librarians. However, they have a big change. They've lost their advantages in Challenges, but now gain Counter-attack and Rage for the player turn whenever they take casualties from enemy shooting, including Overwatch. This is a nice callback to when Black Templars ran at the enemy when they failed Morale checks from shooting. This is much more useful than being better in Challenges, as it affects many more of your models.
Iron Hands: Their Chapter Tactics remained the same, but The Flesh is Weak Feel No Pain roll now stacks on top of other Feel No Pain rolls. Therefore, a Iron Hands Command squad with Apothecary now has Feel No Pain (4+). Not bad, and honestly how it should have worked from the beginning.
Salamanders: Salamanders still get a free master-crafted weapon on all of their characters, and their flamers still re-roll failed to wound and armor penetration rolls. They no longer re-roll failed armor saves against flamer weapons, but instead get Feel No Pain (4+). This is a little worse against anything that can't penetrate their armor, but much better against AP3 flamers, such as the Land Raider Redeemer's Hellstorm Cannons. Standard flamers don't scare Astartes much, but they live in terror of AP3, so I call this change a serious improvement.
Raven Guard: Widely considered the weakest Chapter Tactic in the last Codex, the Raven Guard haven't gotten any better. They can still use their jump packs in both the movement and assault phase, and re-roll failed Hammer of Wrath wounds. They have lost Scout, but now have Shrouded on the first game turn rather than Stealth. This is better for units that plan to stand their ground and shoot, but less so for units that want to close with the enemy. Overall, I don't think it fits the Raven Guard background as much as the old rule. They could have given them Scout and Shrouded and they'd still be one of the weaker Chapter Tactics.