1. SPG AMEX Business for all domestic business expenses and for all Starwood charges.
2. Chase Sapphire Preferred for all business dining and travel, including auto rental. Also for all foreign business expenses.
3. Aadvantage Citi Executive Mastercard for Admirals Club and all foreign personal expenses and shopping.
4. SPG AMEX for all domestic personal shopping and spending.

Occasionally, number 3 and 4 are temporarily exchanged for initial spend to get bonus miles, e.g. Bank of Hawaii Hawaiian Airlines $1000 spend, etc. or when one or another card gives special offer for retention. I've gotten rid of my Diners Club card since BMO took over.