It actually depends for me which game it was on. For Ages, I liked Ricky the most, because of all the cliffs in southern Labrynna, and the worker part with the cliffs was the easiest one, but I like Dimitri more in Seasons. And seeing Seasons as my favourite of the two, I'll go with Dimitri.
Dimitri all the way. I never really liked Moosh or Ricky. Dimitri was awesome because he could like gulp a monster, swim, climb up waterfalls, and his overall design was awesome.
Ricky was cool because he could jump over cliffs and stuff and I liked how he could just jump straight over holes and that he's like a boxing champion.
Moosh I really didn't like because he was slow, he had a pretty good attack, the ground pound thing, but it took a while, and he just hovered over holes, which is sort of cool, but still, Dimitri wins.
Dimitri all the way. I liked how Ricky could do that tornado punch and I also liked the sound he made while jumping. Moosh is okay, because I could hover with him for like one and a half screens, but Dimitri just rules over them all.
I have played through Oracle of Ages several times and I've always gotten Moosh. I really need to go and try out Ricky and Dimitri. I know that one area the workers/carpenters is the only main difference between the characters, but its something that is worth trying out.
I've only played through Oracle of Seasons completely through once, but several times gotten half way or so. I got Moosh again the first time through, and well, that's pretty much it. In general, I prefer flying rather than hopping or swimming.
I mostly like Dimitri because he increases your mobility in water, which is Link's biggest weakness in that game. Moosh and Ricky just go over holes on land, which Link already does well enough with Roc's Feather. They other two may have better attacks, but Link attacks well enough without a companion anyway (especially on overworld enemies).