He loved Apollonia, but I don't think it was (or would have been) a lasting love. I think Kay was THE love his life. Here's why:

(1) Michael and Apollonia's attraction, courtship, and marriage was impulsive. After everything Michael has just been through emotionally, he's practically ripe for getting hit by the thunderbolt.

(2) Michael wasn't looking for an unquestioning mafia wife who understands the way of that world because HE HAD NO INTENTIONS OF BECOMING DON. That was Sonny's inheritance, and Sonny had not yet been killed.

(3) The only reason for Michael's marriage to Apollonia was so that someone Michael cared about deeply -- and only a spouse could fill this role -- could be killed before his eyes by the far-reaching hand of Barzini. This is the required trauma that propels Michael toward becoming Don.

(4) It's notable that FFC changed the novel's depiction of Michael's feelings toward Kay. In the book, Kay seeks out Michael after his return from Siciliy. He does not tell her he loves her even though they marry. In the movie, the fact that Michael seeks out Kay and tells her he loves her points toward Michael's never having forgotten her and having sincere feelings for her.

(5) Michael is troubled by anxieties about losing his family after Kay's miscarriage. He feels responsible, which is why he has a heart-to-heart talk with his mother. That's a very poignant scene, and I don't believe that Michael would have been concerned if he did not love and care about Kay very much.

(6) In that last scene of GF2, Michael is sitting alone, having lost everything he cared about: his father, his mother, Sonny, Fredo, Kay, Tom (through estrangement), etc. If Kay were not the love of his life and he only married her out of expediency, then I think that scene would lose some of its resonance. He's also still wearing his wedding ring.

(7) As much as I dislike bringing up GF3, I have to ask this: How can anyone still not believe that Kay is the love of Michael's life after watching GF3? When Kay tells Michael that she knows he married someone else while he was in Sicily, Michael replies, "But I thought of you." And I believe him. If I recall correctly, in every scene they're in together he's darn near trying to win her back.