My main pairings of units I have been using:
ChromxMaribelle: I wanted to marry Sumia and I felt the other pairings wouldn't work so well so I had him paired up with Maribelle. Not bad just Maribelle is suffering on the magic stat a bit. She does give Chrom a much needed resistance boost though.
MUxSumia: Liked her so much that I gotta marry her. It worked out extremely well. The two of us have been throwing crits left and right like nobody's business.
SullyxStahl: Pretty good team together. They always seem to be able to follow up on each others attack so they tend to take out enemies quick.
LissaxVaike: Axe partners for life!! That's all XD
CordeliaxLon'qu: Wanted to use Cordelia and also wanted to use Lon'qu. Since they can be paired up with each other, I just went with it.
VirionxCherche: They are pretty cute together XD Their support makes them look like an old couple as they all know each others tricks and habits. With the boost of strength from Virion, Cherche dishes out some good damage.
Others pairings I have done:
MirielxRicken: For stat sakes, both magic users and pair bonuses seem to focus on magic and res than anything else. So it was more convenient this way. Also thought the teacher and student relationship would be pretty nice