Savior of KMC
Location: Saving KMC
Sanguinius. Sanguinius, like pretty much all Primarchs, can move fast enough to disappear from the sight of even higher level Space Marines. In Know No Fear, an Ultramarines Captain I believe is able to look at his surroundings and come up with a working tactical contingency plan in a nanosecond, and later in the book another Ultramarine (Thiel, a sergeant I believe) exploits a microsecond-long lapse in the concentration of his Word Bearer opponent (he was distracted in that moment by Guilliman badly wounding Kor Phaeron) to run him through with his sword. Furthermore, in Deus Encarmine, a Blood Angel wields Sanguinius' Spear of Telesto, and in doing so becomes a vessel for the weapon and takes on some of Sanguinius' power, including his speed which allows him to see plasma and las bolts as being nearly suspended in mid-air.
Of course, Black Library is very inconsistent, so it does depend on which author. Of the big three, Abnett Primarchs are relatively grounded, and might lose. Graham McNeill's Primarchs are every bit the mountain-breaking demigods they are in the legends, and most in his stories would steamroll Thanos. Aaron Dembski-Bowden's Primarchs are all over the place, in the same story at one moment being capable of resisting being crushed by Titans, and the next moment being threatened by bolters.
I'd back Sanguinius with the Primarchs' highest feats in mind, and probably also on average.
"I GOTTA FIND OUT WHO KILLED MY DAD"