This thread has helped me a lot so I thought I would try to contribute to the best of my knowledge. The Baltic countries have already been mentioned here, I'll give some more details that I gathered.
Estonia
Trading in as a legal entity is still a compelling option with 20% CIT on distributed profits only. 20% is the final tax rate, there is no other tax of any kind on dividends for a natural person.
Being employed as director and paying yourself a salary is required if you are actively trading. (Investors don't have to, they can stick to dividends.) Tax authorities do investigate companies that only pay dividends to make sure there is no work involved, but they have the reputation for not being too aggressive with penalties.
This salary has to be in line with the average salary one would receive for this sort of activity in Estonia. This salary will be subject to PIT (which is a flat 20% with some deductions that makes it slightly progressive) as well as 38% social security contributions, which are not capped.
Compliance and accounting costs should be lower than in most countries.
The benefit of tax free compounding comes with the risk of a change in tax rates. All the undistributed profits would be subject to this change at time of further distribution. Things do not seem to be shifting in that direction and one might argue that this doesn't happen overnight.
dw31583 mentioned a potential way to mitigate that risk :
Btw, just thinking, how could the Estonian authorities stop you from becoming a non resident once you have millions of accumulated profits and then you relocate to the Bahamas for a few years where you setup an LP in which the Estonian entity is a limited partner for a 0% profit share and you're the general partner for a 100% profit share. In practice what happens is you kept your Estonian company which invested all of its funds to another company in which you as the fund manager gets a 100% profit share. They'd have a hard time challenging this no matter how aggressive this looks like. You can play safer and adjust the numbers but the point is you may be able to mitigate the effects of the reversed corporate income tax of Estonia.
That is not an option anymore. Some new anti-avoidance legislation has been adopted to effectively eliminate all ways to avoid taxation on hidden profit distributions through tax havens.
Trading as a natural person is a possibility but less favorable for obvious reasons. Also not advised if you have a large amount of transactions : due to the details required in tax returns for every trade, it takes forever.
Latvia
Didn't go into great details because I personally don't want to live there. But it should be noted that Latvia has completely changed its CIT. It is now very similar to Estonia. Only distributed profits are taxed at 20%. Dividends are not taxed anymore at the individual level.
Lithuania
It is generally more favorable to trade under your own name. Profits would be subject to PIT : 15% up to 136k€ then 20% (and 27% further down the line, I believe). Social security contributions would not apply.
However, while trading financial derivatives is allowed under your own name, a legal entity is necessary to trade spot forex.
Trading via a legal entity :
5% CIT up to 300k€ turnover (NOT profits), 15% otherwise
BUT dividends are also subject to PIT at 15% for individuals.