Wow, that's a pretty open-ended question. I imagine you'll get a lot of SPG stalwarts telling you how great their program is and how badly Marriott treats its guests. Here you'll get the opposite story. I'm gold with both Hilton and Starwood (used to be diamond with Hilton), and have been platinum with Marriott for many years. For what it's worth, here's my opinion.

Marriott knocks the socks off the competition. Forgetting frequent guest programs for a moment, the entire Marriott chain provides far more consistent excellence in the quality of their properties and customer service than the others. They also have more locations in more diverse places than anyone else. When it comes to the rewards program, Marriott seems more generous than the others with the range of rewards available and the ease of attaining them.

As for elite status, Marriott alone actually seems to reward loyalty. I hear platinum Starwood folks talk about all the suites they get, but as a gold I get crap. Marriott ALWAYS puts me on the executive/concierge floor, and I get suites more often than I probably deserve given the rates I pay. The platinum benefits of a guaranteed room and an arrival gift are quite nice indeed.

So there's my biased opinion. Good luck in your quest to find something better than Hilton--to be honest, that shouldn't be too hard!