Reiner's sexuality is a mystery, but to be honest I dislike the ReinerxBertl pairing because of the seme/uke mechanic in this relationship. This is the reason I like JeanxEren; since they are too similar to each-other, the seme/uke would be nonexistent.
My first favorite pairing was, is and always will be ErenxAnnie. I can absolutely see it more than I can see ErenxMikasa and ArminxAnnie. Though I really don't see ArminxAnnie. Eren and Annie relationship seems to be the most logical. I see Mikasa with either Jean or Levi, though its closer to Jean. I mean, if anyone is able to get Mikasa from obsessing over Eren it would be Jean. Armin might be with Krista/Historia or someone we don't know about. If not, it'll be ReinerxKrista/Historia. I will now post all the pairings I like to see (though some won't due to deaths) in order meaning first is the best while last is the worst. Here it is: