Country or carrier offering most painless prepaid SIM, 'just to have number' anywhere
 

Can be from any country.

Some features might be interesting:

- They ship internationally - no problem if you had to buy the card initially in country (or from eBay or whatever means), but at least they can send you a replacement abroad by mail. (Though it's nice if you can buy and activate original card from abroad as well.)

- Not much if any top-up required - as you will not use this number to make actual calls; just to have a number - any country

- Professional service, maybe a fancy web interface?

- What else might be important to you?