kidnapping. easy, almost always works, and its super fast money (5k ~ per kidnapping) get your thiefs guild to level 2 and build the jail (extra room)

If you upgrade the kidnapping room to the max, you can get higher profile hostages for even more money, IIRC.

Train all of your thieves to 100% stealth skill and focus on robbing only houses. They're generally far less protected than businesses and always have more money lying around (and in the form of cash rather than useless shit you have to manually sell). When scouting, you usually have to do it 3 times minimum (check the ledger and you'll see a percentage indicator of how well scouted the building is) to get it to at least 90% scouted. Once a house/townhouse is 90% scouted, sending in even an apprentice thief with full stealth should be a near guaranteed success. With four thieves in your employ, you should be able to successfully rob two houses per turn on average, which on its own brings in 10k per turn. With a level 3 Thief's Guild, you can double that income (max 8 thieves rather than 4) and boost it even further with regular kidnappings, which should effectively be bringing in 25k per turn assuming you have the patience to micro manage 8 employees at once.

Very profitable, if I do say so myself.

I just played a game as thief. I found the following quite effective.

Phase 1: Establishing a foothold.
1. Train all your thieves to max stealth. Anything worth doing is worth doing right.
2. Case two nearby houses.
3. Discard the less valuable house and send everyone to rob the more valuable one.
4. Repeat from step 2 until you get to the grade where you can upgrade your business.
5. Upgrade your business.
6. Hire and train more thieves.
7. Stop all burglary. Either train your thieves to max combat, get them good weapons, and go for kidnapping, or switch to pickpocketing. The town watch will put a major cramp on your business if you keep trying to do burglary.

Stage II: Taking control
1. Once you have money for it, arm all your thieves with pistols (and train them to max combat if you haven't already)
2. Scout out the lowest level town watch building.
3. ATTACK the town watch building (attack from the crossed swords above the pickpocket bucket)
4. Win the fight, because you have guns and they have swords. Choose to take over the building.
5. Post the building for sale at the highest price you can.
6. Repeat from step 2 until no town watch buildings remain.
7. Go to the local church and buy an indulgence to wipe your criminal record clean.

Stage III: Profit
Go back to burglary. You have the run of the town until the town watch gets the money together to buy their headquarters back from you or build a new one. Whenever that happens, just reconquer and put it back up for sale. The combined income from risk-free robberies and selling the watch buildings back to the watch should take you far ahead of your competition.
If you ever need a breather from the fights, don't sell the watch buildings back; that will give you several years to train up new thieves before the watch gets a clue and builds another building.