I do think Hamer is the best of the 3 and most likely to succeed at the highest level, but like others have said, easiest to replace, for the least amount. Not with a like for like, but with a solid, effective, defensive mid. Like a better, top end Championship version of Liam Kelly (remember when we used to say how important he was to the way we played). E.g. sell him for £8 million, sign a quality CDM for £2million (or a free or high wages), plus another attacking mid who can score goals.
Sure, they won't be as classy as Gus, but that's a luxury we can do without more than Vik's strength, power and ability to hold the ball up and run at defenders. Or O'Hare's constant pressing, chasing opposition down, holding ball up and drawing fouls - making him one of our most effective defensive players by keeping the ball at the other end of the pitch and releasing pressure.
On the other hand, I could make an argument for selling Vik at the peak of his value. He'd probably bring in the most, and could get pushing £20 million if similar transfers for young championship strikers are anything to go by. And as good as he is at creating chances for himself and is important to the way we play, he hasn't been a great goalscorer. His 17 goals this season are skewed by the 9 in 9 he got at the start of the season before teams started to double up on him and get the measure of him. Could he repeat it next season, possibly, but he'd need to improve his chance conversion rate a lot! So while he is an effective target man for us, he might never be worth as much for his goalscoring exploits as he is now, so we could do worse than cashing in on him now and getting the most for him. He'd be hard to replace as he's a unique player, but we'd have to go for different type of players, maybe a couple of fast attackers or wingers, all depends on what's on Robin's radar within our budget.