Right, so now RVP's exit looks to be officially on the way let's look at this properly. Apologies in advance as this post is going to be filled with statistical analysis.
Lacazette is probably the best of the realistic bunch. Plays the most minutes per game, scored the most league goals, the most per game, the most outside the box, has the highest amount of key passes, the lowest amount of shots per goal and highest shot accuracy and is at the best age statistically to have ironed out inconsistencies. His lack of height, physicality and over reliance on his right foot are the only concerns. Also had the highest WhoScored rating of those analysed.
Factoring in physicality, height, brand potential and Premier League experience - Kane would be the ideal pick due to his range of goalscoring attributes and while rarely best, often very good rank in these measures. Also brings the home grown, English quota up.
Personally I don't think Benteke or Lukaku are good enough for United and never will be first team starters in an ideal world but if needs must, Louis could use them but there are many better options.
Of course, if Muller etc are available - muppet mode on - then they are the best option not called Cristiano.