Getting back to drafting top defensemen...
Any metric for defining "top" is arbitrary, but here's one way:
Fantasy top 50 defenseman rankings for 2018-19
And here are where they were drafted (name, round/overall pick, year):
Hedman, 1/2, 2009
Subban, 2/43, 2007
Burns, 1/20, 2003
Byfuglien, 8/245, 2003
Karlsson, 1/15, 2008
Jones, 1/4, 2013
Doughty, 1/2, 2008
Klingberg, 5/131, 2010
Carlson, 1/27, 2008
Josi, 2/38, 2008
Gostisbehere, 3/78, 2012
Hamilton, 1/9, 2011
Krug, undrafted, 2012
Barrie, 3/64, 2009
Dahlin, 1/1, 2018
Letang, 3/62, 2005
Pietrangelo 1/4, 2008
Ellis, 1/11, 2009
McAvoy, 1/14, 2016
Yandle, 4/105, 2005
Provorov, 1/7, 2015
Giordoano, undrafted, 2004
Werenski, 1/8, 2015
Dumba 1/7, 2012
Rielly 1/5, 2012
Ekblad 1/1, 2014
OEL, 1/6, 2009
Ristolainen, 1/8, 2013
Gardiner, 1/17, 2008
Suter 1/7, 2003
Sergachev 1/9, 2016
Muzzin, 5/141, 2007
Montour, 2/55, 2014
S Theodore, 1/26, 2013
E Johnson, 1/1, 2006
C Miller, 5/151, 2012
Parayko, 3/86, 2012
Pulock 1/15, 2013
Faulk, 2/37, 2010
Schultz, 2/43, 2008
Vatanen, 4/106, 2009
Butcher, 5/123, 2013
Green, 1/29, 2004
Shattenkirk, 1/14, 2007
Nurse,1/7, 2013
Matheson, 1/23, 2012
Fowler, 1/12, 2010
Keith, 2/54, 2002
Spurgeon, 6/156, 2008
McDonagh, 1/12, 2007
Of the top 50, number by round:
1st round: 30
2nd round: 6
3rd round: 4
4th round: 2
5th round: 4
6th round: 1
7th round: 0
8th round: 1
Undrafted: 2
Now while the exact list of 50 is absolutely debatable, the statistical breakdown is some pretty strong evidence that the first round is the best place to find a top defenseman.