I mean, the odds of the 1st overall pick being the best player in a single draft class is less than 50/50. In pretty much any scenario I'd take the field. Just looking at recent drafts where the guys have played more than a season. Even top 2 vs the field would be close.
2017: Pettersson is in the lead
2016: Matthews has held firm
2015: McDavid has held firm
2014: Pastrnak or Draisaitl
2013: MacKinnon, but it is not like he kept pole position the entire time
2012: Definitely not Yakupov. Pretty debatable for next with guys like Rielly, Lindholm, etc
2011: Definitely not RNH. It's Kucherov, but Scheifele and Gaudreau have been huge
2010: It's between Hall and Seguin.
2009: Between JT and Hedman
2008: Probably Karlsson, but the top 2 are in the discussion
2007: Kane held
2006: Between Toews and Giroux
2005: Crosby
Outside of a McDavid/Crosby type prospect, it's probably best to take the field against the top 2. It's close. Obviously, with the more recent picks, it is expected the high picks will make the earliest impact, for example, Matthews holds the lead (and I hope he maintains it), but RNH held a similar lead in his first 3 years only to get passed.