The T20 and T23 mediums seem to have pretty decent weapons at least, that 90mm stings a bit already. And got my panther fully upgraded finally, with rammer/stabilizer/vents. Makes for pretty impressive accuracy and rate of fire,combined with the hps you can take out an IS one on one head on if you have to. The biggest downside by far is the repair costs, looks like 0 to 100% is 16000 credits, it adds up fast if you die during a fight.
On the Pz4: Using rammer, vents and camo on mine. It does have a surprisingly high amount of firepower if you know your maps and can get to good locations, the thing really prints money once you get used to it.