Since I would have chosen Riza had she been on the poll, my choice [and in my view, the next best relationship for Roy] would be HughesRoy. However, since Hughes died, it's a tie between RoyLust and RoyOlivier. RoyLust would be interesting; I could see a storyline in which Lust aids Roy in his search for Hughes' killer, because honestly, Lust had plenty of emotion in comparison to Envy; of course, Roy would have to assist her in exchange, and the tension would be interesting. As for RoyOlivier, well, their interactions are interesting and speak for themselves.
Though I also wouldn't say no to a little RoyHavoc.