Each person's situation is different ...depending on where they reside(for hub reasons)..how much of their travel is 1) Hotel stays and 2) Air miles ...some are weighted more heavily on one side than the other thus; the need to dump points onto the lighter side. For me:

Living within hub distance of PHL(US) or EWR(CO)..I chose CO as it's closer to my office. I fly around 100K per year and about 75% of it is domestic.
CO-Primary
NW- Goes in my CO account
HP-Also goes in my CO account
DL- Very rarely...almost always get one of the first 3 with CO being about 80%

Hotels: I get somewhere around 200 nights per year in hotels.
Hilton- Double Dip and HUGE bonuses right now...keep as Hilton points ..but always have option to transfer them out at a bad rate.
Marriott- Used to be my #1 program 200K award was untouchable...they've ruined that! Keep as Marriott points.
Starwood-Good program but not enough properties for me..I stay with them when I get the chance..and during the promos.. Definitely keep as Starpoints.

Car Rental...I get around 200 rental days per year.
Avis- at 50 points per day..Starwood account gets my points here..and 2 or 3 promos per year with double points doesn't hurt. The key for me is that the points are PER DAY...my rentals are usually for the week and 500 miles or so for the week just doesn't cut it..I would rather have 250 Starpoints...my Avis rentals alone get me 3-4 free nights at the 4k level per year..not bad!

Oh yeah...and AMEX points always into an airline account ...usually my CO account too...bad exchange rates on hotels...but here is where some people dump points into the hotel programs due to the fact that they get enough air miles but stays aren't long enough to mount heavy hotel points.
Just my 3 cents