I'm currently (sloooooooowly) grinding my way to a Tiger. I just wanna have that tank in my garage because of its reputation. And the fact that it appears to be a heavily armoured sniper helps too. I do want a KV too at some point and a Hetzer / Stug too.
That said, I do seem to have some troubles with knowing when to actually use High-Explosive rounds instead of Armour-Piercing rounds. The rule of thumb is to use HE when you can't pierce the armour (such as with higher-tier tanks), but I'm having some mixed results. For instance, I have (besides a PKW III Aus. A) a T-46 with the 76mm gun. It has very high damage values, yet slightly lower (proportionally) penetration values. So, I get that using HE is a no-brainer in this case. But in other cases, HE seems to do minimal damage when using it against same-tier tanks.
And while I'm at it: what's a good "money-maker" tank? I hear KV a lot, but the World of Tanks wiki says that the VK 3601 (German Medium Tier VI) is also a good candidate.
HE Rounds are a bit complicated. They do the shown damage if it penetrates, but otherwise it will deal 50% damage. You might already know this, but the penetration parameters for HE rounds are a huge range of numbers. Sometimes it will penetrate against armor it should be penetrating, but usually not.
Some special cases are when the round size is a lot larger then the mm of armor on the enemy tank. If your gun is giant and using HE rounds, and they have relatively pathetic (But still enough to deflect a HE round) armor, your round has a 50% chance of burrowing into the armor and dealing full damage either way. On the other hand, if your using HE rounds against armor that is much, much larger then the round size, even HE rounds will bounce off.
In general, HE rounds should be kept to a minimum unless either your gun is made for HE rounds (Such as the 105mm on the hetz or 152mm on the KV. These are easily spotted by being huge cannons in relation to the tank's tier and having low AP penetration parameters) or there is absolutely no way your going to penetrate.
As for good non-arty cash-making tanks, here are some candidates:
Panzer IV
KV w/107
SU-85 w/107
StuG III
T-34
Easy'8 Sherman
IS-4