At LAX there isn't much different between the major rental agencies in my experience. I would choose whichever has the better rewards bonus.
With a flight at midnight I'd aim to return the car at 10:45. I'd allow 15 minutes to drop it off and shuttle to the airport, then 60 minutes to print a boarding pass, clear security, and walk leisurely to my gate. If I were hauling a lot of baggage that had to be checked I might give myself an extra 15 minutes. If I were driving a long distance to get to LAX I'd allow extra time in case I hit evening traffic (YES, you can get stuck in a traffic jam at 10pm in West LA!) or got lost near the airport.