best player in the 2015 draft 5 years from now...

Porzingis has a lot of upside based on what I've read and seen, but there have been plenty of players that didn't translate to successful NBA careers.
I thought at the #4 pick the Knicks took a calculated risk where they took him, but would have been a silly move for any of the three teams above to take him over the players they took.
The risk seems to be much higher for international players. Going back the past 5 years or so, these are the foreign born players taken in the top 10:

2014:
Wiggins
Embiid
Exum
Stauskas

2013:
Bennett
Len

2012:
none

2011:
Kanter
Tristan Thompson
Valanciunas
Vesely
Biyombo

2010:
Aminu

2009:
Thabeet
Rubio

I'd say there are definitely some busts amongst them, and whilst a lot of these prospects carry a lot of intrigue, there is definitely risk there.
Porzingis sounds like he could be the real deal though. very tall, very athletic, good range/shot, good work ethic. He could be a star.
My pick is Russell. He's going to be a gun on offense when he takes the reins from Kobe.

Hasheem Thabeet was taken ahead of James Harden and Steph Curry (the 2 leading MVP candidates this past season).
GM's tend to reach on bigmen with potential over guys with a lot more proven talent.

Fact is pretty much every team in the league has a quality point guard, whereas quality frontcourt players are few & far between -that's why they take the risk.

Only two players worth comparing Porzingis to imo: Nowitzki & Bargnani. Work ethic is what seperates the two. Both awful defensively, whereas Porzingis has drawn comparisons to John Salley for his shot-blocking.

Devin Booker > Klay Thompson