All three designs are nice (and all are better than STS-134's), but the Omega symbol sells me on the last design. It says final flight better than the above two proposals.
Is this the official patch design of the STS 135. This one looks far better than the other space patches. i really like this one.http://www.collectspace.com/sts135_patch
And you'd be coming back to see Atlantis launch, not Discovery. She's already into retirement.