Did you know that you can totally go first, even after you lose the roll off to go first, or even choose to go second? It's that 6 that changes the game, the groan inducing boxcar, it's Seize the Initiative! Surprisingly there's a lot in this game that modifies this simple mechanic, such as, but not limited to:
1. Callidus Assassin
2. Coteaz
3. Grimnar's War Council
4. The Mirrorcodex (Tau)
5. One Harlequins Warlord Trait (+4!)
6. Imotekh (also the cron Warlord trait)
7. Bjorn
8. Mantis Chapter Tactics
9. Captain Karlaen

Well gee, that's quite a few ways to get first turn! Grimnar's War Council is even better since it's a better modifier than +1, along with a handy benefit to decide who goes first. Ideally we would want to deploy second, gotta have that sweet counter deploy, and then snatch their assured victory from them. The problem is that we then need to do something great with first turn because we're paying such a high tax. For my money I would go with either Drop Pods or Tau, trying to get rid of the chance of the enemy hiding from our sneaky sneaky ambush. Here's the framework I have going in my current dabblings:

Formation Detachment: Grimnar's War Council
Officio Assassinorum Detachment: Callidus Assassin
Tau Combined Arms Detachment:
Commander, Mirrorcodex
Killy stuff to flavor, I like y'vahras and turning the commander into a Buffmander to get rid of as many saves as possible

With this we get a 3+ rerollable seize along with a reroll to first turn. Awesome, though the killy part is still troublesome. Maybe Stormsurges and markerlight support instead? Either way I heard Tau like first turn.

I'm sorry, what? No scatter homing beacon that doesn't have to start on the board?! Hell yes. The big thing here is to also bring it with a framework that could be useful besides, for that I like the Aquila Kill Team Formation:

Librarian, Beacon Angelis
Veteran, Special Weapon of your choice
4x Veterans, Frag Cannons
Drop Pod

Ok, now that we have a turn 1 scatter-less area of effect, let's figure out what to do with it. For me, when I hear turn 1 alpha strike I think SKYHAMMER. *cue the rock*

Skyhammer Annihilation Force (Ultramarines)
2x 5-man Devastator Squads, 4x Grav Cannon, Drop Pod
2x Assault Squads, Jump Packs, Sarge has 2 Lightning Claws

Funny thing, you can bring TWO! So Turn one we elect to bring in both skyhammers, we automatically get the Deathwatch pod, and we start off the game null deployed (pray to go second vs other reserve heavy armies). That gives us the first say by way of 16 grav cannons that are Twin Linked (thanks Devastator Doctrine), along with perfect deep striking assault marines who can charge straight away, along with an awesome 8 Frag Cannon shots to punch void shields or whatever else. Null deploy alpha strike is back! (it isn't)

"The player that set up first can choose to take the first or second turn. If they decide to take the first turn, their opponent can attempt to Seize the Initiative."