The Sergeant York can farm blue stars with no problem even if is matched with higher tier tanks, unless you find an other York player better then you obviously. I feel guilty when I use it as it is too OP, sometimes i farm more then 50% of the whole party damage and kills even in a T6 party, but if I am in a hurry to get those 10 stars is the way i go. Being premium you farm also some good money, even if you play at medium tier.
If you like to get the stars at higher tiers I would say that there is no perfect tank for it, but there are perfect tanks for specific missions. Let's take Harbinger, there is possible to get the star easily with a well armored MBT if you are the one that spots all the tanks for the high dpm tanks camping on the hill, but it is also possible to get it with a GAU on the hill as long as you can spot the enemies coming from the river and do a ton of damge, hopefully also tracking the tanks in the middle so you "steal" assist damage to the MBTs spotting them. In both cases to have the best binoculars and maybe even the upgraded optics is a great help as you spot earlier then the others having more range and with the MBT you also can defend yourself better as you actually spot the tanks firing at you. For the GAU using sabrina is perfect in that mission, some bush camo bonus and improved tracking and engine breaking capability, use the slowest fire setting and short burst so you don't loose too much accuracy and you have an overheated gun much later instead of fire all at once until it is overheated.
For some other missions it is better to use a very high damage tank like the T15, for others a MBT with good dpm and mobility is the better chance, and even LT and gun based TD can work very well for some missions, as tanks like the AS21 and T40 can in others, like Onix or Cerberus.
The blue star is not only a matter of damage, it is a combination of damage, spots (the first one to see a tank), assist damage (damage done by others to a tank that you see for them, have tracked or designated, does not matter if it was spotted earlier by some one that is no more spotting it) and kills. I don't know in which percentages those factors work, but it is possible even to get blue stars with a tank that has problems to do consistent damage like the Crap Jeep on maps like Cavalry if you, with both the view range retros, the stopped camo one and Erin spot for your party most of the enemies, being really careful to avoid that they don't came too close to you ruining your camo and being behind a bush when the lieutenant shows up other way he will out spot you and then you will last very short, as you spot him pop out smoke and retreat behind a bush spotting him and hoping that your mates help you kill him fast.
By the way as the blue star is, or should be, the reward for the better player in the party it is fair if an average player don't get 10 of them in only 10 games, thing that an over the average player picking the right tank for every mission can do, or maybe in 11-12 games as it is always possible to screw something, have something going in an unusual and not predictable way or to find some other one even better then you...
I hope it can help you.
