This is a really hard question, there are so many variables (including how many languages you speak, where you are based and who your partners are). Obviously some functional areas and industries are more conducive to international assignments. In general though, I'd pick the large tier2 companies just because they are so large. Think of Deloitte, ATK, Strategy&, Accenture... and yes, Wyman.
If really this is a huge focus of yours, I'd probably try to focus on niche industries (Oil & Gas industry) or areas.
PS: Who will decide your career path / promotions? Make sure they know you. Often when you are assigned on an international project, you will work with other partners. At some point you need to have a core group of senior folks who have seen you over a variety of cases and can vouch for you. Hard to do it you move around all the time.
Hope this helps -
G, ex-BCG Dallas