Which is the cheapest/best SIM card to buy in Paris?

Following.

The Orange price for Holiday Card is still 39.99 EU.

If you go to an Orange boutique, and still have your SIM, you might be able to ask them to convert this to a Mobicarte, which is a pre-paid system that lets you top-up when you need to.

If this isn't possible, ask to purchase a regular Mobicarte.

Here's the info (en francais) that you'll need: boutique.orange.fr/mobile/recharges-mobicarte
It's a good idea to print any page you think you'll need so everyone is clear about what you want.

Try to go when the store is not just opening, at lunch, or when it's closing time. You'll get much better service and the agent will make sure that your phone works the way you want it to.

If you run out of money on the Mobicarte, you can either charge it by credit card via telephone, or at some ATM machines (I don't recommend doing this on the street).

You will need your actual passport, too, no copies allowed.

Thank you.

You're entirely welcome.

If you want a cheaper card, try Lebara available at most Tabacs and phone stores. They have a website in English to monitor and reup your card if necessary. I don't think you'll be able to use it in the Netherlands so you might have to get a local card there as well. But don't limit yourself to Lebara or Orange. There are other prepaid cards and the deals and prices change often.

I think the initial price of the Mobicarte and Lebara are practically the same.

I stopped in a little electronics store and picked up a SIM for 30 euro. It was three letters SMR (or something). It came with some data and time. I ended up adding another 30 euro before the end of my 2 1/2 week trip. I could have probably saved a lot of that if I had been more judicious with data. I found many times when I thought I had my data off, I didn't (new phone). When I added money, I had to do it at a bar/lotto place a block from where I picked up the SIM.

I'm pretty sure I didn't get the best deal, but it worked fine.