There can be a solid argument for pretty much any of the primarchs to be fair. I just listen to a passage where Horus catches Sanguinius killing one of his own sons that fell to the Red Thirst. Horus explained to Sanguinius that they are just human, they have flaws and they are no different than the humans they were designed to protect. I am sure every primarch has had their moments. It is what really brings them to life and makes them all more than just the killing machines that the Emperor wanted them to be.
An argument you could make for even Magnus. I'm sure he knew what would happen when he used his powers to try and warn the Emperor of the coming betrayal. He sacrificed his entire legion and himself to try and protect the Imperium. Granted I don't know everything about his fall, just that he fell after trying to warn the Emperor.
Now if we were to say that it is the most selfless to up to the current timeline... I would most likely say Guilliman. Look at this poor dude. He awoke to find everything in tatters, a father that views him as a tool of war, and still put himself in a position to carry on the mission he was designed for. As far as I'm aware he has not complained out loud about this position at all. Though I wonder if that will change if one of his trusted brothers returns. I dunno.
I love Sanguinius as well. The Blood Angels have quickly become one of my favorite chapters even if I'm disappointed in their rules in game. I just think that sacrificing himself so they could win a battle, not the war, was a really bad call on his part. It almost feels like the easy way out. Sanguinius thought that the Emperor would not be able to kill Horus without his sacrifice. But that could have been visions that the Chaos Gods were feeding him.
Yeah... I'll be going with Guilliman. The dude just impresses me. Everything I've read about him has only helped to build up Guilliman and tear down this strange anti-ultramarine bais I've had for so many years. You could almost see the stress he was going through at the end of Devastation of Baal when he was talking with Dante. The pain he must have been suffering and the doubt he had to push through to continue to act in the best way possible for the Imperium. Having encountered two of his brothers in the past and now knowing that he was very out-numbered. Dude is a boss.