For making money, currently I'd say KV-5. When it comes to credit earning potential, people always seem to consider the income only, not calculating expenses like repair cost and most importantly
ammo cost. A KV-5 shell costs 250 or 255 credits, every penetrating hit guarantees a 300-ish damage for the price of a cheap shell. Generally on a premium tank, the more shells you fire, the more damage you do, so the income gets higher as you pew pew a lot during a match.
So, when it comes to calculating pure
profit, the KV-5 and the Type 59 come out as winners, Type uses similarly cheap shells, I know the nominal damage is not high (~230), but it has a high bonus to income.
T-34 and Löwe shells cost more than 1k credit per shot. So let's do a quick calculation here:
Löwe:
~320 damage for ~1000 credits
T34:
~400 damage for ~1000 credits
Type 59:
~230 damage for ~250 credits
KV-5:
~300 damage for ~250 credits
On average, let's say you're shooting 20 shells a match. Ammo costs: 20k credits for Löwe and T34, 5k for Type 59 and KV-5. Taking this into account, Löwe and T34 have around 15k less
profit on an average run, just because of the ammo cost.
I have a Type and love it, if I had to choose between Löwe, T34 and KV-5 for moneymaking (and fun!) purposes, I'd hands-down choose the KV-5. It's gun is quite adequate if you know where to aim, and it's always a better feeling to penetrate with an underrated gun of an underrated tank.