Depends how much/often you want to express. If you are going back to work and expressing several times a day or if you are going to exclusively pumping then you need a double electric pump (something like a Medela Pump in Style or a Ameda Lactaline) or even a hospital grade pump like a Medela Symphony or a Ameda Elite.
If you just want to pump occasionally then a good manual pump like a Medela Harmony or a Avent Isis or Lansinoh manual. I'd stay away from the Tommee Tippee pumps as I've never read a good word about them and I used a manual one but never managed to get a drop out.
I've got a Philips Avent electric single pump and it is blimmin' fantastic. A bit noisy perhaps but comfortable, easy to put together and use and effective. I always get loads of milk out. My first one broke but that was my fault. Bought an emergency tommee tippee which was all they had in the 24 hour Tescos but hated it - never got as much out as I did with the Philips. So I bought another one and use it at least six times a day as I express nearly all my baby's feeds, so it gets some punishment.
It works on mains power or AA batteries so is portable. Also you can control the speed - you start off pumping at the speed that suits you, then press the button and the motor "remembers" what you did and carries on at that speed.
One of the best things I ever invested in (no, I don't work for Philips!)
Really depends on how often. Before work I got on fine with both manual (tommee) and electric (medela swing). Not much difference in yield, but I think a lot of it comes down to how well the shield matched your breast and nipple shape/size. In the end I used both of them at the same time. Electic won on producing a consistent amount but manual won on being quiet (handy when you have to express out and about, not that many people have that need e.g. weddings). Plus the manual is so cheap, there's nothing to lose.
I've used a hospital grade (double pump) and it's been great as went back to work and also DD went through an incredibly fussy period beforehand. More that it's just loads easier and quicker (manual made my hand hurt, but I have small hands and single electric is longer as you have keep switching sides). It's also quieter, but bulky and costs about 30 pounds a month.