Sicarans are a very popular choice in people's armies, so that is probably a safe pick.
For loyalist armies with Primarch models, I like the Ultra Marines. Excellent rules, lots of foot soldiers is fluffy, and they look great.
For Traitor Armies with a Primarch, I love the Reds of the word bearers. Again, foot soldiers is fluffy. I think they need 2
HQ choices, so make sure you model a chaplain and a praetor.
Otherwise, I would say Imperial Fists. I believe that deep down, everyone is an Imperial Fists player but is afraid of painting yellow.