I like Astorath the best for a jump infantry style approach. I think he gives you the most bang for the points. Mephiston is very popular but I think smart opponents won't have much trouble killing him plus there is a lot of anti psyker units in the game now.
I really like both Honor Guard & Sanguinary Guard and also think you should load them up with lots of killy toys. Here is how I run mine:
Sanguinary Guard/Banner (+1A), 2x infernus pistol, power fist
Honor Guard/power fist, power sword, meltagun, infernus pistol
I take one Sanguinary Priest (lightning claw, infernus pistol) and attach him to the SG. Astorath runs with the Honor Guard.
Astorath & his unit have reroll hits when charging, furious charge & FNP.
Priest & his squad have furious charge & FNP.
These units crush face. They are small so i run them in Stormravens for protection & speed... Stormravens are awesome! Mine have extra armor, TL lascannon & TL multi-melta plus the 4x Bloodstrike missiles. It's a lot of points tied up in a relatively low number of models but that's how I roll!
I am also running Furioso Librarian & regular dread (multi-melta). They are also attached to the Stormravens!
Finally I have two full assault squads with jump packs - meltagun, flamer, infernus pistol & power fist
it's a small elite army that can alpha strike. I don't see many people running SG or HG but these are awesome units. I used to run two 5 man veteran assault squads (3x power fist & 2x meltagun) and they did just great... The two units I mentioned here are a lot tighter. I have lots of experience playng BA, having stuck with them during the PDF, so I think I am ahead of the curve.
I really like the Furioso Librarian, not that much more than a Librarian for the points plus he is a lot tougher (front armor 13) and he can fly using Wings of Sanguinius.
I am not really all that worried versus IG Mech or most any gunline army. You've got to be smart about it but it shouldn't be a problem.