What job gives the highest reward when requesting help/backup from your mafia?

I believe the maximum is for the Shake Down a City Council Member job, which is in the Boss job tier and opens up at level 120. It is also the highest experience job for you to do by yourself in the game. I will verify this in a day or two when my own help timer is refreshed. :)

• Helpers get 13 experience and $2,400,000
• The requester gets 124 experience and $72,000,000

In general, it looks like a helper will get roughly 1/10th the base experience* of a job and you will get a bit less than base experience when the job is complete. Shake Down a City Council Member is roughly 137 base experience, so each of the 10 helpers gets 13 experience, and you'll get 124 xp once the job is complete. For a job like Blackmail the District Attorney which is roughly 69 experience, helpers get 7 experience and you'll receive 66 experience.

See What's up with reqesting help on jobs? for more general information on requesting backup.