DTW is the biggest and most junior, but has the most choices of aircraft. MEM is pretty small, just 9's and 320's. ANC is just 747-200 cargo. SEA is a 330 base only and pretty small. MSP has all types except 744. They are standing up a small 747-200 base in MSP for mil charters. Don't know how long that will last. Here's some rough numbers of Captain Positions at each base. FO numbers will be a little higher than the capt numbers.

ANC- 70-747
DTW- 250-A320; 160-A330; 160-744; 210-757; 150-DC9
MEM- 85- A320; 50-DC-9
MSP- 320-A320; 25-A330; 200-757; 120-DC-9
SEA- 90-A330

SEA is the most senior (I believe). We send new hires everywhere except SEA. MEM is senior for the limited jets it has.