As I have previously mentioned, there are so many different methods for integrating Roboute Guilliman, especially given that he can be taken as a LOW by any Imperium faction, that I find myself continually exploring more possibilities with him.
As I have thought about it, one army that I think he would pair particularly well with is the Black Templars. Just reviewing, Black Templar arguably have some of the weaker Chapter Tactics for competitive play, with Crusader/Adamantium Will on all infantry, the ability to gain Rage and Counterattack for a turn if they suffer any casualties, and a restriction that prevents them from taking any Librarians.
To compensate somewhat for these “so-so” Chapter Tactics, Black Templar do get access to the unique Crusader squad as a Troops choice, which is basically a more flexible, more capable Tactical Squad in almost every respect (the only thing Tactical Squads get over Crusader squads is the Combat Squads special rule).
In place of this, Crusader squads can take an additional PW, PF, or Heavy Weapon in the squad, no matter the size, can have its squad members trade out Bolters for CCW for free, and can take 10 point Neophtyes (basically Scouts, but without Infiltrate/Scout/Move through Cover) equal in number to how many Initiates (i.e. “normal” Marines) are in the squad. Finally, the squad can take the normal Special Weapon option, can upgrade one member to a Sword Brother (i.e. Veteran Sergeant, with all the normal options), and can take either a Rhino, Razorback, Drop Pod, or Land Raider Crusader as a DT. Taking all this together, you have quite a versatile Troops choice that can be customized multiple different ways, including for close combat, MSU shooting, mechanized assault, or even giant “blob squad” foot squads of 20 men each (i.e. 10 Initiates, 10 Neophytes).
Now that we have gotten an overview of Crusader squads, I think Guilliman can integrate with them in two major ways:
 Close Combat Infantry Horde — If you choose to go for large Crusader squads on foot, the Guilliman can be in the center of the line and buff them in multiple ways, including giving them Ld10 (with re-rolls for Pinning/Fear/Morale checks), Move through Cover, +1″ to their Run and Charge moves, and re-rolls of 1s to Hit for both shooting and assault. With good positioning and Guilliman’s 12″ buffing radius, you can have multiple 20 man Crusader squads benefitting from all these buffs, which in turn makes them more mobile and more deadly in both shooting and close combat. In particular, the combination of Move through Cover, +1″ to Run distances, and Crusader means that even footslogging infantry can get across the board quickly. Combined with the cheap Neophytes and Black Templar forces can potentially replicate Orky “horde” tactics with masses of Zealous Astartes (i.e. the infamous “Black Tide” tactics of 4th edition). As an added benefit, Guilliman can serve as a “fire magnet” for enemy heavy weapons to draw attention away from Crusader squads, further enhancing their ability to get across the tabletop and safely into close combat (or, if they shoot at the Crusader squads, then Guilliman is left unharmed and can wreck havoc in close combat himself, so either way it is a losing proposition for your opponent).
 Mechanized MSU Force — With the ability to take a Special Weapon and a Heavy Weapon in a minimum 5 man unit, Crusader squads are an excellent choice for loading up in Rhinos and using them as mobile bunkers, with their two firing points, to put down lots of protected firepower each turn. Guilliman can serve as the “anchor” for just such a mechanized force, both buffing the shooting of all friendly squads/vehicles within 12″ and providing a devastating counter-assault element if enemy forces come too close themselves. This can make for a very strong “mid-field presence” force that can shoot, assault, and (importantly) take lots of Objective while still keeping pressure on the opponent’s army. This also works quite well with lots of Dreadnought support, since Guilliman can buff them in the same way he would enhance the “black tide” style army mentioned above and also provide highly effective close combat capabilities alongside them (i.e. massed S10 AP2 attacks at full initiative from both the Dreadnoughts and the Primarch). Finally, if you decide to go with massed Land Raider Crusaders for your Crusader squads, then you can force your opponent to choose between engaging them (and thus ignoring Guilliman as he footslogs across the battlefield) or ignoring the Land Raiders and trying to deal with Guilliman (and thus leaving himself open to massed assault by the Crusader squads riding inside)… either way, it poses a significant dilemma that can be used to your tactical advantage as a Black Templar player.
Besides making different styles of Crusader squad based armies more effective, Guilliman can also enhance the “supporting elements” in a Black Templar army, especially fire support elements like Predators, Devastators, TFCs, etc. One unit in particular that I think benefits from Guilliman being in the army is the Stormhawk Interceptor, especially when taken in mass (you can take three of these in a CAD and up to six of them if you run the Castellans of the Imperium special detachment). With BS4, re-rolls to Hit of 1s (when within 12″ of the Primarch), and the ability to put out 2 Lascannon shots, 4 Assault Cannon shots, and 2 Krak or Frag missiles a turn (with the option to shoot at enemy flyers/FMCs at full BS) from a single airframe, these Astartes flyers can be exceptional fire support platforms for a Crusader squad heavy Black Templar force, further reinforcing the value that Guilliman brings to such an army.
Based on all this, I think Guilliman really can be a tremendous asset in a Black Templar army and maximize their value of their unique units, while also mitigating their somewhat mediocre Chapter Tactics.