I liked all of them as a character excluding later Kalos Ash but it's a tie between Ash in Hoenn and Alola. Of course I loved how Ash was in BW, always happy and energized, he could take a joke, and knew when it was time to butt heads. He had so much chemistry with Iris and Dento and it was fun to make fun of his shortcomings but he was always so adorable here. OS Ash was funny but he was always kind of a dick and really immature that he was annoying at times but it was heartwarming how he tries to be there for his Pokémon, how compassionate he is when one is in need.
I loved how in Hoenn, Ash was starting to mature. He could be calm and chill but when you pushed his buttons, he was ready to fight so he was rash in that area. He was kind of a mix between maturing to still having some of his old traits back in OS... and not the good kind lol. In Sinnoh, my heart couldn't take how emotional, upset, angry, at his wits' end, annoyed, stressed, and all types of sides Ash brought to the table. You really felt those immense feelings from beginning to end. When he overcame this obstacle, you could feel the sense of pride, joy, and triumph experienced.
In Kalos, I liked his earlier bromance with Clemont and his positive altitude to motivate everyone. I voted for Ash in Alola.