From my experience...

Lounges
Best - Silver Kris Lounge (Singapore)
Worst - Premier Lounge (Singapore)
Almost Nonexistant - Los Angeles International

Reservations
Best - United (well for Premier Executive at least)

Loyalty Program
For Regular J/F - Singapore Airlines PPS Club
For Mere Mortals - United Airlines Mileage Plus

Y / J / F Cabins and Seating
Best First - Singapore Airlines
Best Business - Singapore Airlines (though Air New Zealand might trump it later this fall)
Best Economy - Singapore Airlines (on A345s)

Food / Drink
Best - Singapore Airlines
Improving - United Airlines

Customer Service
No one really stand out. Maybe United because of my status.

Baggage Handling
No one stands out.

Routes on own Metal
Best - Singapore Airlines

In-flight Entertainment / Communications
Best - Singapore Airlines (hands down)