It's Jack Hughes.
But, let's keep in mind, that even if the Sens are the worst team in the NHL (far from a lock), they have a higher chance of drafting 4th overall than drafting in the top 3. Brady is probably one of the most divisive prospects around. Personally, I'm a fan of his game (although, soon enough the Leafs fan in me will hate him, not only for being a Sens but a complete shit disturber) and had him in the 3rd tier of the draft, clearly behind Dahlin and Svechnikov, but with Zadina and Dobson. I'd probably take Turcotte, Cozens, and Kakko ahead of him at the moment, but its a long season to see if they can maintain their standings. There's also a good chance I'd lean Podkolzin at the moment, although, the Sens don't draft Russians, and he hasn't played much above u-18 hockey in Russia, outside of IIHF events. Past that, I think I'd lean Brady at the moment over the next group of guys like Byram, Dach, Krebs, Newhook, etc. So, to make it a reasonable decision to pass on Tkachuk in my eyes, you would have to expect to finish last or 2nd last. While that might be reasonable in the NHL, there is still a bunch of volatility. We've seen the last two teams to finish 30th make the playoffs the next year (Leafs and Avs), the Devils, a team projected to be bad last year made the playoffs. Now, I'm not sure where the Sens can find the growth required that fueled these 3 examples, but, I don't think its unreasonable to see them finish in the 4th to 10th last range, which makes passing on Tkachuk extremely risky.