This is a no-brainer:

Engineers

or Money if you REALLY need money.

But in most cases you should choose the Engineer Rewards in the early game. You need 10 x (number of already existing Satellite Uplinks) Engineers to build one Satellite Uplink.

Here is a random picture from the Internet to visualize the example:

Let's pretent the abductions take place in India, United Kingdom and Brazil and we don't have any satellites ready.

We will choose to rescue India. Reasons:

• We won't get the South America continent bonus ("We have ways") anyway since Argentina left so we can abandon Brazil as well.
  
• The panic in Europe is too low to be relevant at the moment.
  
• If we don't choose India, China will leave the XCOM Project.

Game difficulty is not a factor in panic increase:

The panic is decreased by 3 in the country where the mission took place and unchanged in the rest of the continent.

The panic is increased by 2-3 in every other country where the abductions took place and
1-2 in the rest of the continent.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
THIS part is VERY important

Now how do we know which country to choose and which one to ignore?

We don't want any country to leave the XCOM Project. That means we always have satellites ready for the worst case where RNGesus left us.

Most people choose the country with the most panic which is wrong in some cases. There could be already 2 countries with a panic level of 4 in the same continent as a low-panic continent where an abduction takes place. Not choosing this country will cause the other two leave the XCOM project at the end of the month.

If reaching a panic level of 5 for one country is unavoidable you should prioritize countries for abduction missions and satellite launches that are not in the same continent as your XCOM base. You already have that continent bonus so you can let them go to hell if unavoidable.



If no country is in danger to leave the XCOM project even if abduction missions are available and it's not early in the game you should choose the country that gives you the best rewards (Engineers or §, whatever you need the most. Even soldiers can be an option if the soldier has a class that you currently need because somehow the game gave you only snipers).

• Abduction missions are available for 30 hours in Enemy Unknown.
  
• Only if a satellite is deployed over a council nation at the first day of the month or earlier it won't be chosen for an alien abduction mission in that month or later.
  
• There must be at least 2 council nations without satellite coverage (at the beginning of the month) for alien abductions to happen and they will happen twice a month until you successfully raid the alien base. Then the requirement is 3 council nations without satellite coverage and if it's the case then only 1 alien abduction will happen every month.
  
  
• The following enemy types won't appear in alien abduction missions:
  
  • Chryssalids (and therefore Zombies, too)
    
  • Ethereals
    
  • Outsiders
    
  • Sectopods
  Sectoid Commanders may only appear in company with Mectoids