Metal Slug is one of those games that remind you of how good a game is when work is put into it.  Superior hand drawn 2D sprites with a ridiculous amount of detail pretty much sums up the work put into this game.  It's basically Contra with an unbelievable amount of graphic detail. 

Metal Slug Anthology, which came out for the PS2 back in 2007 was a collection of Metal Slug 1 - 6.  This allowed fans who played this in arcades to be able to enjoy all the arcade hits all in one disc.  The bad news was this sold out pretty quick that year and today, this copy fetches up to $60+ for a brand new sealed copy. 

This is for the original version, not the "FAVORITES" which is no as sought after if you are also a collector.