You are discounting Harden here. He was a top 10 player out of HS and had a monster 2 years of college. He exited ASU with career averages of 19 PPG, 4 AST, 5 REB, 2 STL and shot 50% from the floor and 38% from 3 on 4 attempts per game. His production, given the volume of outside shots he was taking, was elite.
It remains to be seen if Doncic can be efficient at higher USG levels. Harden was sustaining those numbers playing 35 minutes a game. While I'm not suggesting the EuroLeague is not on par with PAC12 basketball, Doncic's numbers are in a little under 20 min a game, so it's tougher to see how those additional minutes will drag down his efficiency.
I like him a lot as a player but I just don't see the equation of potential here (league MVP). Just because players have similar athletic profiles, does not mean they have similar ceilings. Harden was uber efficient playing 15 more minutes a game against quality college competition, with the pedigree of being a highly ranked HS player. He also drew FT's at a rate exponentially higher than Doncic (5.8 per game vs 2.2) while also displaying the ability to finish consistently against larger, more athletic players.
Im not saying Doncic has a low ceiling but I'm far more inclined to bet on athletes who develop vs. lower athletic ceiling guys when betting on superstar potential. I'm not saying Doncic WON'T be a star, I would just be a little more skeptical on him reaching that League MVP type ceiling given his limitations athletically.
The above wouldn't stop me from taking him #1 because I think he's a really safe bet to be (at minimum) a good starter but there are guys who will go behind him that have lower floors but higher ceilings IMO.
I think you're underestimating the difference in level of competition between college and Euroleague/ACB.
Here are Euroleague/ACB stats for players who've been drafted in the lottery the last few years:
Bender averaged 2 points, 1 board, and 1 assists in 11 minutes per game (17/18 years old)
Papagiannis averaged 2 points, 2 boards, and 0 assists in 5 minutes per game (18 years old)
Porzingis averaged 11 points, 5 boards, and 0 assists in 21 minutes per game (19 years old)
Hezonja averaged 6 points, 2 boards, and 1 assist in 15 minutes per game (19/20 years old)
Saric averaged 12 points, 6 boards, and 1 assist in 22 minutes per game (19 years old)
Obviously not all of these guys have panned out, but my point is they wouldn't even be on the radar of NBA teams if they were putting up those numbers in college ball. Especially notable are the low assist numbers; Saric, Hezonja, and Bender were all billed as guys with great feel for the game and some point guard skills. None of them even managed 2 assists per game in Euroleague/ACB play.
Doncic's in a completely different ballpark putting up the numbers he is against such a high level of competition.