>>32803116I'd say best dad could be narrowed down to Koga, Birch, Stone, Byron and Juniper.
The other dads barely even see their kids being away from home for extended periods of time. I'm pretty sure Elm's wife complains about this specifically despite him living and working in the same town.
And considering 4 of the 5 mentioned have grown children and we don't get to see much parental interaction, if any for them I'd say based on what we see of all of these fathers raising their kids, Birch could be considered best dad.
He spends time with his kid, gives them their starter Pokemon, teaches them and has them assist his research and is generally a pretty cool dude.
To be fair though I haven't finished Sun and Moon, but from what I've seen so far Mohn seems to live on those bean islands and Kukui is fathering Lillie and Gladion is running around with Team Skull. 
