It really depends on what you are factoring in.
I'll say its Ottawa for a couple of smaller reasons which I'll start with, and then get to the major one. Firstly, the Senators really only have two pieces on the current roster, which is Tkachuk and Chabot. Past that, everything else is pretty replaceable or aging. Guys like Brown and Pageau have impressive numbers given their track record, but they are getting an abundance of ice-time not making their stats particularly efficent. They have some nice prospects in the NHL/AHL bubble-like Batherson, Norris, and Brannstrom, but I wouldn't call either a clear blue-chip to build around. Finally, they have two likely top 10 picks, which need to be converted on, because as pointed out in the first point, they lack core pieces, primarily a 1st and 2nd line center (although, maybe Norris fills one of these roles).
On paper that isn't terrible, you can argue other teams are in worse shape when it comes to that stuff such as Detroit, or a team is capped out with an struggling aging team such as SJ, a team that has had promising starts turn into terrible 2nd halves like Buffalo or a team like Minnesota who lack exciting young core pieces on the roster. At the end of the day, it comes down to the elephent in the room. Which is, Eugene Melnyk. There are bad owners who seem clueless (like the Pegula's, Katz, Aquellini), and then there is the Eugene Melnyk level of dysfunctional ownership. He manages to botch getting a heavily subsidized stadium, he is getting sued from multiple angles for not paying debts, and is just in general has a reputation for being a pretty sleazy person. Would I rank Ottawa last if they got new ownership tomorrow? Probably not. But, given how stubborn Melnyk is, he's going to drive this team to the bottom before he is somehow forced to sell either by going broke or finally doing something to piss the league off enough. Yeah, where teams like Detroit, SJ, and Minnesota are isn't great, but they at least appear to have somewhat competent ownership (especially SJ), and while the Pegula's running of the Sabres has been bad, I don't get the impression if they ever luck into the proper hire for that team that there actions will actively undermine the team (see the Bills and McDermott). Whereas Melnyk's cheapness and interventionist ways are sure to undermine Ottawa at some point.
I do think MTL could be in trouble. They define directionless right now, and as the francophone population continues to diminish, they are just further handicapping themselves by their outdated rules. I mean, Gallant gets fired and half of Habs nation is googling if he can speak French. Now that Briesbois has been promoted, who is the best french-speaking candidate for GM?