Buenos Aires
Yes, it is worth every penny and every minute. The flight is not so bad. You leave Europe at night and arrive in B. A. the following morning - no jet lag. It is a fantastic city with lots to see and do. You won't get bored.
Argentina is a huge country with plenty of variety and you could easily spend months travelling around. Still, it is worth going all that way just to spend time in Buenos Aires and at the moment your euro will go a long way there.