The entire financial sector. It captures 30% of all corporate profits in the world economy. Ads are bad, but not "take 1/3 of what other people make" bad.
They're also (belatedly and sometimes decreasingly) heavily regulated in most of the world. What they do isn't always rent-seeking.
They charge you for not having enough money in your checking account, while using your money to give out loans and make money off of that. Basically charging you for being poor while making money off of your by fee and loaning your money to other people.
I'm old enough to remember they never charge a fee for not having enough money in the bank account. I left my bank for a credit union.
Also Well Fargo was caught recently in a dubious sketchy scheme.
It's a very complex systematic problem. I'm not saying burn all banks let's move to the jungle and eat coconuts.
Mob bosses own legitimate businesses. They're still crooks.
edit: Just FYI: I am not anti-bank. I am anti "capture 1/3 of all economic profits". No economy can survive that sort of parasitic activity in the long term, regardless of the intention behind it.
If you're willing to "steal" from other companies, why wouldn't you also "steal" from the owners of your own company? Why pay a dividend when you can pay bonuses?