bb - all valid arguments. It could be anyone really but I would focus on these four in the order of priority:
1. An alum that you have directly worked with (former supervisor or senior manager/leader in your company) - this is hard and you have to be really lucky to have someone like this.
2. A senior leader in your company who may not have been to the school you are applying to but graduated from a top tier MBA program.
3. Alums that you have networked with throughout the admission process.
4. Current students that you have networked with throughout the admission process.