Ok Noo - I'm Mrs Anal when it comes to car seats - its a well known fact around the other childminders in my area!!! I have done soo much research and even spent a small fortune for a Which? subscription so I could read their test results on all the car seats!!!
I agree with old mummy - other road users are total idiots and your car seat could be the difference between serious and not serious...sorry to sound a bit scaremongering!
I went with Which?'s best buy reccomendations for safety with all the car seats I have had (and I have loads!). Like Musswells and others, I LOVE the maxi cosi Tobi - safe, idiot proof when it comes to installing it and yes ok expensive - but very very worth it. I also have a Britax Renaissance (although these are harder to find these days...) which is fab as it will fit with just a lapbelt so you can use it in the middle of your car if you want to. Both seats go in solid as they have a metal "foot" that comes out the bottom of the seat after fitting to make the belt go rigid tight. Brilliant.
Don't get one that goes up to 11 - Which? are quite critical of combination car seats as each stage is unique and needs different safety features. Stick with 9 months to 4 years. Go onto manufacturers websites - I know that Britax.com do a fitfinder search where you type in your car make and model and then it will tell you which seats will in theory fit in your car - will save you alot of time when you go to visit Halfords and Mothercare!!!
Go to both by the way as I have found they have different opinions as to what is a good fit in your car. (well my local stores do anyway!!)
Finally if you want to save a bit of money (and apologies to anyone who works in halfords/mothercare!!!) I always go to both stores, get them to fit the seats I'm interested in, show me how to do it and then I tell them I'm going away to think about it....and order the one I've chosen online from eg Kiddicare or somewhere like that.

Naughty I know but I have a total of 9 car seats for various ages and stages due to my job and I have found I've saved a fortune doing this and I haven't compromised on safety as I've checked they definately fit first!!
Good luck!
Me 31
DP 25
DS1 - 5
DS2 - Born 22.5.08