Ash x Latias
He is practically dense when it comes to other girls, But Latias is almost completely devoted to him through the entire movie (And I still attest that it was Latias that kissed Ash in that last scene). Why not have an interspecies relationship? =D
Brock x Lucy
Lucy is one of the very, very few females that's actually head over heels for him, and on that note, the only one that Brock didn't freak out about (In the get-away-from-me way like he did with a few previous interested females). In fact in 'The Queen of the Serpentine', she remembers him fondly at the end, and Brock says while on the boat 'Lucy. Forever in my heart.'
May x Drew
Jessie x James
Once more, 'nuff said
Mew x Mewtwo
The game may say they're genderless, but Mew is quite very feminine, while Mewtwo is rather masculine. Rough start, but I think they'd balance each other out.
Max x Gardevoir
Max saved Ralts and promised to return for her. No doubt his relationship with her will grow much stronger as they grow together as a team, once he's old enough that is.