I know no official discovery order of the Primarchs has been revealed, but i want to make a list which is as accurate as possible as to the order in which the Primarchs were discovered (with your help of course
Heres some facts we know:
1. Horus was discovered first.
2. Alpharius Omegon was discovered last.
3. Perturabo was discovered before Angron and Guilliman. (Source: After Desh'ea)
4. Magnus was discovered before Lorgar. (as he, alongside the Emperor discovered Lorgar)
5. Fulgrim was discovered around 160 years before the Horus Heresy as his flagship was built then. (Source: Fulgrim)
6. Fulgrim was discovered before Konrad Curze.
7. Jaghatai Khan was discovered before the Lion. (The Dark Angels first mission (Post-Primarch Discovery) was taking over from the White Scars)
8. The Lion was discovered roughly 148 years after the start of the Great Crusade. (Fallen Angels is set in the 203rd Year, and also set 50 years after Descent of Angels.)
9. Rogal Dorn was discovered seventh according to the Horus Heresy Chapbook.
10. According to 'Fulgrim' the gun Ferrus Manus made for Vulkan was forged 200 years prior to Isstvan III (ish), therefore both Ferrus Manus and Vulkan would have been discovered 200 years prior to the Heresy, which would make them two of the first found, although I fear this may have just been a mistake by Graham McNeill.
11. All the Primarchs were discovered prior to the Ullanor campaign. (Source: Legion)
12. Vulkan was discovered before Angron. (Source: After Desh'ea)
13. Guilliman was discovered before Angron. (Source: After Desh'ea)
14. Mortarion, Corax and Dorn were discovered before Magnus (Source: A Thousand Sons Page 385-387)
15. Magnus implied to have been discovered before Fulgrim, Ferrus Manus & Lorgar - although not conclusive evidence (Source: A Thousand Sons Page 96)
16. Magnus, the Khan and Sanguinius discovered before the Lion (Source: A Thousand Sons Page 322)
Where no information exists on certain primarchs we can logically assume their discovery order in accordance to there homeworlds distance from Terra. From this we can assume that following Horus; Magnus, Russ and Sanguinius were some of the early discoveries. (Although this is not solid evidence).
In no Strict Order...
Personal Speculated First 5 (1-5): Horus, Perturabo, Fulgrim, The Khan, Corax.
Personal Speculated Next 5 (6-10): Russ, Rogal Dorn, Guilliman, Mortarion, Magnus.
Personal Speculated Next 5 (11-15): The Lion,
Personal Speculated Last 5 (16-20): Alpharius, Night Haunter
Anyone have any more information?! (+Rep to those who can find or knows any relavent information!
Here is the table so far:
1. Horus Lupercal of the Luna Wolves.
7. Rogal Dorn of the Imperial Fists.
18. Alpharius Omegon of the Alpha Legion.
(Ive only included 18, as its pointless including the Unknown Primarchs in this!