I really like almost all of the companions in the game that I've traveled with. So far I've maxed out the affinity for Piper, Cait, Curie, Codsworth, Preston, and MacCready. I've traveled a good amount with both Nick and Paladin Danse, though I haven't maxed out their affinity yet. I haven't traveled with Deacon yet aside from the first mission that you do with him. I rescued Strong but haven't traveled with him at all yet. Aside from Preston, I really like all of them so far. Preston just annoys me. His facial expressions are totally blank and his dialog is incredibly dry. Plus I just have a grudge against him for all of the Minutemen radiant quests lol. I sent him to the Castle, ignored the last couple, failed them, and haven't talked to Preston since. I probably won't for a while.
It's a toss up for me. I really like all of the rest. Piper, Curie, Cait, and MacCready are my favorites. If I had to narrow it down, I'd have to say Piper and MacCready top the list. I've romanced Piper, Curie, and Cait, but Piper is still my 'main' romantic interest. MacCready is basically my 'best friend' kind of companion.