The new sixth edition rules allow all armies to take Allies. The five Space Marine Armies are almost all Battle Brothers with each other (as per the fluff Dark Angels and Wolves are not). Since this means they treat each other as friendly units there is little reason to not take allies (points permitting). There is a slight cost since you have to take a HQ and one more troop unit from the second codex in order to unlock an extra Heavy, Elite, Troops and Fast Attack slot.
The only restriction is you can not use each others Transport vehicles; thinking back about all my 5th edition games though I can hardly ever remember a game where a unit got back into a transport, let alone one that was not theirs originally.
Still having an old 4th Edtion Codex Templers have some good options with 6 man laz-plas units and Terminators with Tank Hunter. On the flip side they still have 50 point Rhino's so they are the only army that would kind of like to borrow someone’s transport they also will not ally themselves with any army containers Psykers except for Grey Knights.
You can take a Castellan in Terminator Armour for just under 100 points, although the upgrade to Marshall for an extra wound and Leadership 10 to all your Templer units is a good buy at 15 points as it spending a few more points on better weapons.
Both the Terminator Command Squad and Elite Terminator's get to take 2 heavy weapons with 5 man squads and can take a Land Raider Crusader as a dedicated Transport as well as have access do the Tank Hunter Skill. With only one entry there is not many options in Troops, you can add cheap scouts (10 points each) to the troops units to have some extra bodies to soak up wounds.
Black Templers did have cheap Typhoon Speeders since the 5th Edition FAQ upgraded them inline with other Marines to be Heavy 2; unfortunatly the 6th Edition FAQ has removed that secion so they are back to being Heavy 1 Frag Blast. Hopefully this was a mistake as a squadron of 70 point Typhoons would be a good addition.
Triple Lascannon Predators are actually cheaper than their counterparts (145 vs 165) and AutoLas Predators are only 5 points more expensive, Vindicators 10 points. All of which can have Power of the Machine Spirit added. While expensive a Vindicator moving 12” and still shooting could be quite a surprise to your opponents...
98 – Castellan with Tank Hunter TDA and Storm Bolter
128 – Marshall with Tank Hunter, TDA, Power Sword and Storm Bolter
217 – 4 Command Squad Terminator with Tank Hunter, 2x Assault Cannons, 1 Chain Fist.
270– 5 Elite Terminator's with Tank Hunter, 2x Cyclone Missile Laungers, 1 Chain Fist
151 – 5 Initiates (Marines) 3x Bolters, 1 Lascannon and 1 Plasma gun in a Rhino.
155 – Vindicator with Power of the Machine Spirit.
With the changes to characters and the addition of Warlord traits as well as getting Victory Points for killing the enemy leader having HQ that are both killey and survivable has become useful. Outside of Librarians the generic leadership options are all a bit weak, that does still leave us Mephison, Dante and Astrorath.
Mephison is still a beast giving you a level 3 Psyker with access to the new powers. While the powers are random the chance of having a Toughness 8 Eternal Warrior or an Invisible Mephiston let alone the ability to re-roll all his misses in combat may make it worth taking the chance...
Dante is still good bringing hit and run and no scatter to his unit but being saddled with an Axe so he is only striking at Initiative one is a real bind. Furious Charge does put him at Str 6 AP 2, with 6 Attacks on the Charge. Sanguine Guard as Troops is another benefit although I am not sure how much use they are without the army being focused on Jumpers; they cost as much as Terminators with out the survivability of an invulnerable save.
Astrorath got better now most melee weapons do not ignore his 2+ armour save. Given that his Axe is a unique weapon he also gets to keep his Initiative 5 and is also Strength 6 AP 3 in combat. While probably a mistake he is also the only character in the Codex that did not have is rules restricted to only work on Blood Angels; so for the time being he will give any Marines he is with Fearless and re-rolls to hit on the charge. While not a massive boost it could be useful in the last few turns of a game.
Of course the main reason to allied with the Vampires is to have Assault Marines (ASM) as troops so they score. I loved my Chaplain and Ultramarine ASM in 4th Edition but not being able to hold an objective in 5th resulted in them staying in the box gathering dust, now they can finally come out to play again. ASM's in a Razorback are also a good option; as are scouts if you want a minimum priced unit to fill in required slots who can also go to ground in a ruin for a 2+ cover save making them hard to remove..
165 – ASM x5 With Meltagun in Plasmaback
225 – ASM x10 with 2xMeltagun, Sgt with Bolt Pistol & Plasma Pistol
090 - Scouts x5 with Sniper Rifles and Camo Cloaks.
Dark Angels are the other Marines with a 4th Edition Codex, although rumoured to be the next one to be replaced. Many of there units are just weaker than the Ultramarine options so the best they have to offer is Scoring Terminator by taking Belial as your Allied HQ. The nice thing about Deathwing Terminator's is you can mix Hammer/Shiled and shooty terminator together and even put the missile Launcher on a model with a Storm Shield and/or the Sergeant so you can use precise shot and Look out Sir!
130 – Belial with Storm Shiled and Hammer.
260 – Deathwing Terminator Squad with Apothecary and Cyclone Missile Launcher
230 – Deathwing Terminator Squad with Cyclone Missile Launcher
075 – Typhoon Land Speeder.
Wolves are going to be the most common ally taken simply because there psykers still have the power to counter other psi powers including blessings. In fact they are about the only unit in the game that can still do so! Grey Hunters are cheap can have melta or flamer in small squads and you can attach the Wolf Guard to be a Sergeant if you want. The other use for the Elite slot is the Lone Wolf; A battle mad Terminator with two ablative wounds (wolves). The heavy slot offers you an extra Predator if you want to spam AV13 or the excellent Long Fang for missile launchers.
100 – Rune Priest
155 – Grey Hunters x5 with Meltagun and Plasmaback
140 – Long Fangs x6 with 5x Missiles
So what do the mighty Ultramarine's bring to the party? They have cheap Storm Shields; Librarians with Null Zone and Gate; Tactical squads with heavy weapons; Sternguard; A better Ravenwing than Dark Angels, and if you really want you can just take a cheap unit of scouts to fill the troops slot. They also have Combat Tactics which got a big boost in 6th since the presence of nearby enemy does not stop units from rallying any longer; therefore so long as your fall back move kept you on the table you will automatically rally in your movement phase.
140 – Librarian with TDA and Storm Shield
400 – Assault Terminators x10 with Hammer and Shield
210 – Tactical Marines x10 with Meltagun & Missiles (or Multi-melta) in Rhino
200 – Captain on a Bike with Relic Blade, Storm Shield, Artifact Armour, Melta Bombs
210 – Bikes x 5 with Meltagun x2 and MM Attack Bike