Besides the Usual Shady Characters like Redd, who do you think is most likely

[LilD]

Sahara. She sells rare items from around the world and has many connections. Could easily see her involved with illegal trading of stolen goods.

 

[DubiousDelphine]

LYLE: Wild world was a scam insurance. Also a HHA person in NL
LElia: Selling things too cheap
Able sisters?: where do they get the clothes. What secrets do sable hold as she doesn't talk with you for the first few days you meet her.
Timmy and Tommy: What do you expect from the adopted sons of tom nook.
Gracie: TOO EXPENSIVE
Tortimer: HOW DID HE BUY AND OWN AN ISLAND, Maybe by the boonbox donations in WW?

 

[Chipl95]

https://www.youtube.com/watch?v=EPJfYMgqlq4

There is a bit in this video that makes me think Tom Nook could be in a mafia or even be a Don.

 

[aburayacrossing]

You said except nook, but dude. That guy is as close to a ACNL villian as we are going to get. Although he doesn't need to pull a shady underground deal. It's all cruelly above-board. "Your loan is half a million bells, no problem, right? " CAPITALIST SWINE!