We use a Cosco Scenera for travel - it was cheap and it's light and easy to install. No frills, but we've found it perfect for use in planes, rental cars, and taxis. Haven't found an x-ray tunnel that it doesn't fit through. Haven't found a seat it doesn't fit into.

I also recommend this product which lets you wear it on your back like a backpack. I've made it across large hubs with the baby in one arm, carryon/diaper bag in the other, and car seat on my back. We don't use the bag to check the car seat - just for easy transport. (Only side note - the car seat in the bag doesn't fit through certain hand luggage x-ray tunnels. We have to remove the car seat from the bag at security and then put it back in).