Robbery set to High Roller. There's a guide from GTAF all about Empire Building that does show that Robbery High Roller businesses generate the most money.
I believe it's somewhere in the $120,000 to $150,000 range. Could be more or less but I'm not sure.
Yeah, they do. From that Empire Building guide, you get three times the normal income from that Empire after getting the highest rank in that Empire's mission (Example: High Roller Robbery gives you $1,400 before doing any Robbery missions. When completing all 6, you get three times the income from all Robbery missions you have so you get $4,200 of income for every High Roller Robbery Empire you have so if you have all 30 Empires set to Robbery, you get a maximum of $126,000 a day).