Fulgrim for me. But Angron is a close second. Easy to see how both primarchs fell. Also for Fulgrim, I love his legions' paint scheme, and if I was going to support a Chaos god, it'd be Slaanesh or Khorne and not Nurgle or Tzeentch.
Fulgrim
Perturabo
Konrad Curze
Angron
Mortarion
Magnus the Red
Horus
Lorgar
Alpharius
Fulgrim for me. But Angron is a close second. Easy to see how both primarchs fell. Also for Fulgrim, I love his legions' paint scheme, and if I was going to support a Chaos god, it'd be Slaanesh or Khorne and not Nurgle or Tzeentch.