I would argue that the best marines with breacher shields are probably going to be drawn from from either the Iron Hands or Imperial Fists due the Rights of War that can help these units out or their own special abilities (included here [for example] are the Immortals of the IH with Bitter Iron to make them Troops, Hatred and Stubborn in the enemy deployment zone)
For special occasions there are also the Mournival Events only unit - XIIth (World Eaters) Legion Triarii Breachers which come with all the usual gear plus chainaxe, FnP 6+ and Counter-attack
So, which do you prefer?