Swaghmar,
I have been thinking about this question for quite a while myself. A lot depends on what you wish to get from a FF program.
If you want to EARN miles, I think the best programs are NW and DL. Every paid flight earns full miles. NW is better if one is elite; more bonus miles (50% bonus for silver vs. 25% on DL for same).
It's more difficult to earn miles on most other airlines:
UA- Beyond Germany, one flies on LH, and earns only 50% of flown miles.
BA- Only certain class of fares (S,K,B & M) earn miles. But, BA won't give them on some K,B & M fares. Won't tell you ahead of time either.
Most of the European airlines' FF programs have similar fare class restrictions, which limit your earning capability on those discouted tix in economy.
On the pacific side, until recently, SQ, TG and CX didn't give miles (or gave less miles) for the discounted economy tix. Might have changed recently.

ELITE LEVELS
For elite benefits, NW is horrendous.
1) Can't upgrade with miles unless you are paying (nearly) full fare. Extremely rare episodes of upgrades to WBC on status alone have been reported.
2) No lounge access to elites on those long flights.
For elite benefits, DL is slightly better. A few people I know (Silver & Gold) have occasionally been upgraded to/from India, on status alone. I think upgrading using miles is tough here also. Can't comment on lounge access for elites.
BA elite levels are treated well. Nice lounge access, but MUCH harder to get to the first level with the same level of flying/spending.

When SPENDING miles, I have had decent success with NW (about half the time, got seats/dates we could use; used rulebuster option otherwise).
Have had less success on DL. Dropped them eventually.
BA great success. Got dates/class I wanted nearly every time.

Want Randy and company's opinion on this? check out their reply to my mileage makeover request this month.