I like 2.1.3 because the planet view is so much nicer with the tiles, 1 pop per tile, 1 building per tile, and the planet can hold as many pops as it has tiles, simple and intuitive.
Me too, I stick with version 2.1.3 because it's the most playable and enjoyable for me. I love the pops on planet tiles just for the groove of it. And I absolutely agree that it is simple and intuitive.
Also the fleet system (that wasn't implemented prior to 2.0 or so) I think is quite neat.
Though I'm still not getting over to miss the three different FTL drive possibilities from the pre 2.0 version. But I guess you can't have everything.
Considering all the versions 2.1.4 and higher ... there Stellaris is no longer a game for me, honestly. I have given up writing my opinion on Paradox forums since I supposet the devs will do whatever they want to do anyhow.
The last DLC that really convinced me was Utopia. After that I felt there was a real decline both in concept and story quality.
The whole planetary destruction mechanic being intruduced with Apocalypse DLC ... personally, I consider it to be a kind of Jar Jar Abramsification of Stellaris. But, admittedly, the game wasn't so much hard scifi even before neither.
As for the new planet system post 2.1.4: I think it is bulky, unintuitive and unnecessary. Also I don't like the new ressource system. But that is just my personal opinion, of course.