What are my rights if mechanics lien is filed against me by a sub but GC was paid in full and didn't fully pay subs

I am sorry to hear you are the victim of an unscrupulous gc. In Maryland, the mechanic's lien law will actually require an owner to pay out twice on commercial work, meaning, payment to a gc is not a defense to a subcontractor's lien.  

But, you do have several defenses, for example, as you already mentioned, if the overall work of the gc did not improve the value of the building by 15%, it is not subject to a mechanic's lien at all. You may also have defenses based on the timliness and sufficiency of both the notice and the actual lien petition.  

You also have to ensure you file a timely and complete verified answer to the lien petiton. Bottom line:: you need an experienced Maryland construction lawyer immediately!

Please feel free to reach out to me directly to further discuss how I can be of assistance: joe@joekatzlaw.com.

Good luck!  

If the contractor truly has not met the 15% requirement, then you have a pretty good defense there. There are other commons defenses to evaluate as well, such as making sure the notice you received conforms with the laws of Maryland.