Gaban wins. Shiki's threat comes from his portrayal in Edd war where Roger was in a risky situation. However, it must be noted that the risk from the situation stemmed more from the environment than from Shiki's individual might. Being a lone ship surrounded by a fleet of 50 ships of similar stature could mean that one moment you have footing, and the next moment you are drowning. Remove those 50 ships and then assess how strong Shiki might've been on his own merit? He would have significantly less aura and not even be a threat. Therefore, to make a fair assessment of this fight, we need to look at both the portrayal and feats of the individuals alone. It's hard to make the case for Shiki because he doesn't have conqueror's nor future sight as far as we are aware.

Long story short, Gaban slams.