If you're concerned that any direct challenging of a bitchy remark will be met with defensiveness then just raise your eyebrows instead, look surprised and unimpressed by the comment and take a second or two before saying "ok..." or "right..." or "thank you..." then smile and change the subject.
React as though you're obviously taken aback by your friend's lack of manners but are sweetly and graciously glossing over their embarrassingly poor social skills :) And make sure you change the subject in a breezy "right, lets talk about something else" way.
It will make you look and feel more poised and confident than your so called friend. Once you've done this a few times, ie refused to ignore their rudeness but not "got into it" with them they'll probably stop doing it as it just isn't as satisfying for them anymore.
It may even make them feel slightly embarassed because they'll be aware that they've made themselves look like a nasty, uncouth twat and haven't even had the payoff of you getting upset about the put down.
They make the comments because they see you as less confident than them and therefore, in their bitchy competitive little head, they feel superior to you.
Ignoring the behaviour altogether pleases the so called friend because they see your acceptance of their put downs as weakness and it confirms their feelings of superiority towards you.
And challenging the comment delights them too because they feel that they've got to you AND they then get to have another pop about you being over sensitive, can't take a joke etc.
So much better to "obviously" rise above it, so they can see that you're rising above what is childish, unpleasant behaviour from them.
I do realise that this approach is a very, very passive aggressive one but it's honestly the most effective and least stressful way I've ever found of dealing with this kind of shittiness.
Best of luck OP. You sound like a lovely, genuine person and you don't deserve to be treated this way.