I'm not sure about "the best" but I've been pondering a switch myself. Here is what I see in terms of good vs bad:

Good:
Upgrades -- They seem to be easier to come by, especially for the top couple of tiers. I do have a question for you guys though regarding ex-West Coast upgrades. As someone who does most of my flying to/from SAN/SNA/LAX, I wonder if those are good markets for any FF program.
Promos -- Grand Slam is the best promo of any FF program period. If you are willing to spend a little money, you can get a lot out of that promo. The threads here help a lot too.
Employees -- Aside from PHL, I have found US employees as a whole to be very nice.

Bad:
PHL -- No explanation needed.
Award Tickets -- You can't modify the return origin after leaving on your trip. Most other airlines will allow this for a fee. You also can't change partner tickets period once the outbound is flown. This is a huge negative IMO.
No 1 Way Tickets -- AA and UA have them. US and DL don't. I use them often with AA and UA. I will often end up booking 1 way on UA and 1 way on AA just to get the availability I need.
Award Change Fees -- $150 for the simplest change (or is it $75 at US?). I'm sorry but it costs them nothing to change the date on an award ticket so why tax the user so much? Only people exempt are CP.
$25 Award Processing Fee -- Nobody is exempt and it is per ticket. Sure it's revenue for US but I think it's wrong to nickel and dime elites.
3 Tier Pricing on Award Tickets -- Very hard to find low tier awards.

As you can see, for me most of the issues for me are involved in award redemption. That is something I'm very concerned about. Why have all these miles accumulated if you can never use them?

Every FF program has its good and bad. I'm searching for the one with the bad that bothers me least. I like upgrades and I like being able to use my miles. No program really seems to offer both..