Couple of things before I get into the topic...
i. Credit to Silas for pointing out Huntelaar years ago. Yes, years ago. He's definitely been his biggest fan and biggest supporter in wanting him to come to United.
ii. The thread was started because it definitely is an area which needs to be strengthen. Sir Alex has pointed this out more than once.
iii. Has Manucho's permit been sorted? Has he shown any real progress during his loan stint to instantly command the 3rd striker spot?
Now onto the choices and those mentioned.
I'll immediately rule out Ashton and Berbatov.
Ashton because he simply can't keep fit. He definitely isn't better than Saha and if United are looking to improve the squad, why bring him in?
Berbatov is self explanatory. As the founder of "The Keep Berbatov Away From Untied" Campaign, his behaviour over the course of hte season is reason enough for the campaign to be started in the first place. United have done an excellent job of having a group of players that support the cause of winning trophies. Not only do they do that, they support each other. Bervatov simply hasn't done that with Tottenham. He is a me first player. And to me, this simply outweighs the game he brings to the table. He can piss off to a team in Europe and sulk there.
Santa Cruz is out for me, because he is injury prone. He had a fabulous season, but keep in mind he left Bayern Munich to get starts. How much starts would he get with United to be happy? Furthermore, he isn't better than Saha.
Fabiano, I've only seen play once and really don't think its fair to give an opinion. Hopefully someone on here who follows him can weight in.
Doyle and Kitson?

[I'm going to pretend I missed that discussion...]
To me, I'm hoping Benzema or Huntelaar come and here's why:
1. Age - They are at an age where Sir Alex can balance starts with the striking group. It also keeps United's philosophy of mixing veterans with talented younth in line and most importantly, it allows them to properly get there feet wet in EPL competition.
Obvioiusly Benezema and Huntelaar are experienced players. But playing in Holland and France are completely different to playing in England.
2. Game compatibility - Benezema links up play well, is fluid off the ball in finding space and he has the ability to interchange positions. From up front to wide. Very important in United's game nowadays.
Klaas, although a stationary striker up front, is very deadly in finding seams and openings off the ball. He is a clinical finisher. The goals don't lie and he has scored a bunch of them. Can head the ball exceptionally well. My only knock with him from the matches I've seen him play is he doesn't involve himself enough in play. He just waits till the end to score.
United need a striker who:
a. can score at a high rate. On par or higher than Rooney or Tevez
b. can latch on to crosses with the head.
c. can interchange roles with Ronaldo, Nani/Giggs, Rooney/Tevez
d. is able to effectively link up play and keep with up with the uptempo attack.
Benzema is capable of this now and I think that Huntelaar could be taught how. Either choice would work for me.
NOTE: At one point my hatred for Huntelaar was massive. I swore up and down I saw Ruud van Nistelrooy. Attitude included. My stance has soften, my loathing for Ruud has evaporated and seeing this season unfold and wondering whether Huntelaar could make a different. I firmly believe so now.