i have 4 carseats - my car, husbands, nana's and babysitters..... they all ended up being different - and by far the best one is the Britax Marathon i have in my car - she's been in it since she was 9 mos old. It was the most expensive - but worth every penny for ease of use - it is the easiest to adjust 5 point harness, latch clips are the best (similar to a regular seat belt buckle) i can take it in and out myself (i've taken it out to clean/vaccum my car, put in girlfriends car, adjust the harness straps to the next height) and the marathon can hold up to 65lbs. it sits more upright so she has no problems looking out the window.
2nd car seat Graco Comfort sport in my husbands car - we didn't get this until she was like 15 mos old (husband got rid of pickup and got a maxima). this one is really big and she looks like a peanut - but i bought it b/c when she's old enough we can use it with the regular seat belt. there is this extra piece for under the butt/legs (gracopedic support) sometimes gets in the way. but low profile sides allow her to get in and out by herself.
Safety 1st - Uptown - this is in the babysitters car - it's comfy enough - decent padding.... straps i find twist alot
Cosco Scenera - least amount of padding - only used once a week in nana's car....
the biggest thing is the latch system and the hooks that are used for every car seat but the Britax brand. i find them very hard to adjust - to pull the strap to tighten there is just no easy angle to get it to pull tight enough - and find it's usally a 2 person job - one person to push down on the seat and one person to pull the strap to tighten.
I'm having a second baby in July.... and plan to get the britax when we're ready.