I just wrote a list of country from the Wikipedia ISO 3166-1
My goal is to use it in select inside my xhtml, but it's a huge list. How many are you from Aruba? But how many are you from United Kingdom, France, or even United States of America?
Sometimes, I can see that the most common countries are listed at the top of the list.
Is it a good practice? Why and why not?
Why: For the exact reason you mentioned; if you expect most/all of your users to be one country, listing it first will help the user experience.
Why not: Every one not in the most populous countries may wonder why those countries were listed first (almost like a form of privilege). But if you don't want to worry about that you do not have to. Plus, users can still start typing their country in, and locate it that way, so this may not be a big deal.
There may also be solutions to figure out where the user is sitting, but I'm not sure about that. If there are, that may be the best of all worlds.
This is a web application. I'm not sure if any sites do it, but you could sort the list alphabetically, and use IP Geolocation to guess a country to put as the default. It won't be right 100% of the time, but it might make filling out the form easier for the people you do get right...
My opinion is to stick to alphabetic order; I believe almost everyone starts typing the first letters of their country as they open the drop down. :-)
Think of your common web user... It's easier for someone (who doesn't know they can just type the first letter of their country and find it fast that way) to find his country in an alphabetically sorted list then a list sorted arbitrarily based on web population.
I recommend sticking to alphabetical sorting.
Depending on your target audience, it can be either convenient or offensive. I know that I personally find the practice mildly insulting. In addition, it somewhat breaks the ability to quickly select items by typing the first few letters - when everyone sorts it the same way, when you type the same thing, you get the same result. But if you put some countries on the top (and different people do it for different countries), then results may be different.
You can show the full list in alphabetical order and set the country that you expect more users to be from as the option selected by default.
List them alphabetically, and duplicate your primary markets at the top.
Don't just move them to the top - I've wasted too much time looking for the United Kingdom under "U" in drop down menus for that.
List them alphabetically. Even us Americans ought to understand that.
However, I do recommend you consider something a bit fancier, like a text box with an auto-suggest list. You could add a dropdown arrow at the side for those who like to scan through lists, though I suspect that many of them will use the keyboard to at least get to the vicinity of the country they're looking for).
OBTW, I presume you got the English-language list of countries?
I'd agree that alphabelically is the most universally understood mechanism. Custom sorting will most likely cause some users confusion.
Will the application be localized in other languages? Make sure to take into account that the list should be alphabetically sorted based on the users languages.