So here we go through the pros and cons of each. This is not to rule any of them out, as I don't think you can at this point, but to lay it all out there.
Hacktivists (Specifically New World Hackers):
Pro - claimed responsibility. Anonymous/offshoots responsible for lots of past DDoS activity.
Cons - Several security firms called BS on the evidence, and cited past history of false claims of responsibility to boost DDoS for hire business. Also the complexity and sophistication make this unlikely.
Pro - probable originators of Mirai botnet, likely responsible for preceding DDoSes of Brian Krebs and OVH.
Con - No stated ransom demands (at least none reported) or other identifiable material benefit. Lacks a direct reason.
Pro - Past history of DDoS and malware attacks. Never claims responsibility. Suffers nothing if the internet goes down.
Cons - Attack only targeted the USA, not perennial NK targets of South Korea or Japan. If this was North Korea, why ignore those two?
Pro - contacts/influence in Russian cybercrime community. Possible interest in interference in US politics.
Con - No real rhyme or reason for doing so now. Widespread (as opposed to targeted) disruptions likely don't have any predictable impact to swaying the election.
Pro - Reports that many of the infected devices were Chinese in origin
Con - China normally steals your business secrets rather than DDoS you. Chinese devices weren't the only ones, too - bad security is everywhere.
US intelligence (NSA et al)
Pro - False flag?
Con - NSA wants to listen in on your data, not shut you off from communicating. Unlikely that there is anyone who supports Wikileaks/Assange/Anonymous/etc that would change their minds over this.
This is by no means a comprehensive list, just off the top of my head.