Like others have mentioned, there is no single answer. It depends on how you earn, how/where you'd like to burn, what kind of hotel you'd otherwise book in a world without hotel points, what kind of vacations you take (e.g., lots of short weekends to big cities, longer resort stays, etc.), and what elite status levels you can attain within each program.

The only somewhat objective places to look for baseline values are Diner's Club and Amex. They offer conversions from their own currencies into all three of the big hotel programs. Unless changes have been recently made, their ratios are 1:3:4 for SPG:MR:HH.

That passes the smell test for me: I usually peg 'em at 3 cents, 1 cent, and 3/4ths of a cent. Looking at my recent redemptions in all of the programs, and comparing to what I would have been willing to spend on a decent hotel room for those stays, it roughly jives. Sometimes I'm tempted to peg SPG a little higher, but then I ask myself "Honestly, would I have really paid $349/nt. for that room? No."

I tend to do my paid business stays at Marriott, with Hilton as a backup.

I use my SPG Amex is my primary-spend credit card, with just enough HH Amex usage (a couple months a year) to lock in Gold status for the next year.

If I need more HH points, I know I can get 'em quickly from my AA account. The AA -> HH conversion is a roughly equal one to me, provided I'm doing a big enough transaction to minimize the impact of the $25 vigorish.

Learn 'em all, play 'em all. The more diversity you have, the more likely you'll be able to find an award room when you want it where you want it.