If your only options are Leviathan, Ramuh or the Allagan Grimoire then Ramuh's has the highest stats, since it's the only i100 out of those (unless you upgrade a Wave).
As an additional note; you mentioned that your secondary job is Scholar. Make sure you're using the stat bonuses on Intelligence when you play Summoner because if all you have in the foreseeable future for Summoner is an i100 weapon and on top of that you have your points on anything but INT your DPS is going to be lacking, making the bonus of having an extra raise not very appealing.