The bolded one by far. Rashy is far more proven/better than Grealish, Haaland is better than Kane. Sterling is still better than Sancho though.
I'd compare Rashford to Sterling and Sancho to Grealish tbh, given the roles they'll have. Sancho and Grealish the creators, Sterling and Rashford wide forwards. Though also the chance that it's foden instead of Sterling like last year, but still he'd be a wide forward and not the creator anyway.
Currently Sancho is more proven and IMO a higher potential, but with a similar current level (Grealish just showed it for a much shorter period). Rashford when in form and fit is on par with Sterling, but showed it far more infrequently than Sterling (while both are ahead of Foden IMO). Kane more creator than Haaland, Haaland way better long term due to age but also the pace, power and ruthlessness.
Can't only look at them individually though, and Uniteds potential one with Haaland would compliment each other far more IMO. Grealish, Kane and anyone but Sterling in their front 3 leaves it with everyone liking the ball to feet and dropping short, no threat in behind, no pace in attack. With Sterling, he'd be the only guy who does it with the other 2 loving to drop into the pockets as a 10 essentially. Works, but still I think too reliant on Sterlings running in behind. With us, we'd have the creativity of Sancho who can give width on the right and come short and build up/create, but then Haaland being a beast in behind with constant runs and Rashford being a bit of a dual threat running in behind to support Haaland and give options to Sancho (and those behind), but also able to drop deep and create to balance it.
Of course, that's reliant on getting Haaland. But I'd happily give City Kane and Lukaku to Chelsea if it means we get Haaland, without a second of hesitation.