If you're looking to Sign a new Player to your Football Club, one of the more important things you should consider is how they fit into your overall scheme. If a player is going to force you to alter the Tactic your club using on the field of play, then it might not be the Best thing to sign said player. It's also important that a player has the key attributes for the position you're looking to play them at, as, for example, a central midfielder who isn't skilled at passing is practically useless.
On top of how a Player fits the scheme you are running with your team and the having the key attributes for the position they will be playing for your ball club, it's also extremely important to compare a player's physical attributes to the Tactic you are using before signing said player. If you're running an extremely fast strategy with your team, you will need a player that has very good pace attributes. If they don't, you might want to look elsewhere for a potential fit.
So, in short, there is no one statistic that is more important than any other. On the contrary, it truly all rests solely upon the position you are trying to fill within your team and the necessary Technical, Mental and Physical Attributes needed to fulfill said position and the given scheme you run within your team. Should a Player fit the perfect mold, having all the necessary skills you are looking for with a given position, then definitely look to add said player to your team if you don't already have him. If you do already have a player like this on your team, then you even might want to consider starting him (if you aren't doing so already).