I've got a Maclaren Quest and I really like it. It's pretty easy to take up the steps with The Boy in if a section of the escalator is closed or something and it's umbrella fold so fits into most cars. I see a lot of people with Combi's as well, I think they're quite light but look a bit plasticy for my taste, then there's the ubiquitous Bugaboo - great looks but quite small for some todders and I think they had issues with quality (wheels or brakes, I think) but I think that might be sorted with the new model.
Eugene and Bumps have fairly wide selection if you want to go along and test them for weight, ease of use, folding etc.. I think the lightest one you can get is the Maclaren Volo but it doesn't have much recline so not suitable for a newborn.
We've got a britax car seat, agree with Carang that if they've got all the right safety stuff they all seem pretty similar.