Your Bastard scales C/C where as the Old Knight is A/E if your dex is higher like that it will be your best option as it gets better scaling hence better damage. But without exact numbers it is hard to say is this is actually the case. But your best bets for greatswords are the Claymore and Mastodon.My mind is full of fu** with the stats of this game.
So i've decided to use greatswords and i hace these maximiced
http://darksouls2.wiki.fextralife.com/Old+Knight+Greatsword#.U3qP_Pl_vuM
http://darksouls2.wiki.fextralife.com/Black+Dragon+Greatsword#.U3qSK_l_vuM
http://darksouls2.wiki.fextralife.com/Drangleic+Sword#.U3qSQPl_vuM
http://darksouls2.wiki.fextralife.com/Bastard+Sword#.U3qSTPl_vuM
Black Dragon Greatsword is shit, because it doesn't scale, in theory, Drangleic sword is better than Bastard sword but i'm doing more damage with bastard sword than Drangleic...
Old Knight greatsword in theory should be my best option but, again, bastard sword makes more damage i think.
My build by now is 35 strength and 25 dexterity