In BTD6, Monkey Bank. 2/3/0 is best crosspath. Personally would start off with 2/0/0 to get some extra early income quickly and then 2/3/0 to get the Bank upgrade. Lots of 2/3/0 Banks is great. You could also use 4/2/0 Banana Research Facilities too, but you'll need some decent hand micro in order to collect those crates so regularly.
The first path will make $320 on every round (if it is 3-0-0). However, unlike the other 2 paths, you have to collect each banana yourself. As Qwerty said, the 4th upgrade is a really viable way to make money, producing $1500 every round.
The second path will only make $276 on the first round (including the 15%).(0-3-0) But, due to the 15% Interest it makes, It will quickly make more money than the first as the rounds go on, as long as you don't collect the money every round. You have to collect this from the bank every now and again. However, in my opinion, I don't find the 4th upgrade ability very useful.
The third path will make $240 per round. (0-0-3) It doesn't do anything special to increase it, though there is no form of collecting you need to do, it goes straight to your balance. It's 4th upgrade gives you $900 when the round ends, making $1200 (0-0-4). Though it does buff merchantmen's (0-0-3 Monkey Bucaneer) money production, so if you have a lot of these, it's probably better.
Overall I wouldn't recommend the bottom path, unless you have a lot of merchantmen. Though I'm not sure if the buff for merchantmen stack with each Central Market you buy. If not, you wouldn't ever need more than one.
Update: With the most recent update, Central Markets buff on Merchantmen can now stack, and up to 10 times. Each buff to Marchantman is 10%, so you get a 100% buff. If you are merchantman spamming i'd say it's pretty good!
This was my attempt at using the buff, along with trying out the Dark Knight’s new ability. (The alchemists were there cause i got bored lol)
With those, I was getting about 55-65k a round, not including the Monkey Bank.