Answer:  We get this question a lot.  First I would start with an era.  By far, the most popular Lugers are the Nazi era guns (WW2). However, I have people who don't want anything from the Nazi era (for obvious reasons) so for them I would recommend a WW1 luger. Now that we know the time period of interest, this leads to the TYPE Luger you want.  The most popular Type luger would be military issue with military proofs on the gun.   So in WW1 it would be a German military issued gun and in WW2 era (most popular) it would be a Nazi issued and proofed gun.   The commercial Lugers from the pre-war period as well as the between war period are NOT as popular and harder to resell when the time comes to divest.   Also, post WW2 lugers are much less desirable.  Following the military luger in popularity would be the Police issued guns.  There are various types of Police guns and Legacy Collectibles can walk you through those choice should you want to go that route.

Now lets narrow it down a bit more.  If I want the most popular era and type, then I am looking for a Nazi era, military issue.   Within that group there are several options.  The most unique and therefore VERY popular is the "Black Widow" luger.   These are among the last of the Lugers made - 1941 and 1942 only.   Production ended to make way for the improve design of the P.38 in 1942. (Krieghoff Lugers are an exception as they were made right up to the end of the war - Krieghoffs are a whole different breed of Luger, and are equally as collectible).  The Black Widow gets its name from the black plastic grips and magazine bottom.   This makes for a very "dark" and sinister looking gun.  Some collectors proposed that these were SS guns, due to the all black look.  But in truth there is no connection to the SS.  They were issued to the German military.  They only evolved to black plastic as it was cheaper and more durable than the previous wood grip and aluminum bottoms.  
Other than the Black Widow, as an investment gun I would recommend excellent condition and all matching numbers.  Finish must be original and 2 matching magazines is the ultimate, but rarely found in real (not boosted/faked) condition.  The modern renumbered magazines are more and more prevalent.  So much so that whenever I see a gun advertised with 2 matching magazines I assume 1 or both are "boosted".  Buyer beware and please contact us for a free opinion on a high end gun before you purchase.   Of course we want you to buy from us as we give a life time authenticity guarantee on the guns we sell, but we also regularly advise our customers on which guns look correct and which are not.
In summary, buy popular and all original quality Lugers in order to get the best return on your investment.  This is not only true for Lugers, but in any collectible piece of history.  Feel free to contact us for help with collectible firearms.