There are so many cat carriers on the market that it is hard to choose which would be the best.  I would never opt for a cardboard one as they are too fragile if you have an active or frightened cat and they are not waterproof either - so any accidents and you are left with a nasty, soggy mess.

I would always advise you to buy the biggest size of cat carrier you can as cats do not really like to be in them, but will be more comfortable if they are not squashed !!  Also, a larger one will probably accommodate 2 cats and if they are ok travelling together they tend to comfort each other on the journey.  Just be careful when transporting by hand as if both cats go to the same end of the cat carrier it will tilt !!  

Always make sure that the cat carriers have plenty of ventilation as a hot cat is not happy and certainly not comfortable.

Lastly, if you are using the cat carrier to travel by air you will have to follow the IATA guidelines before purchasing.

If you are buying