Dark Souls is the highpoint of the series for many fans, myself included (although it was my first Souls game so I'm biased). Dark Souls is the spiritual successor to Demon Souls and was released on Xbox 360 in 2011 and then on PC in 2012, although the PC port was pretty bad (it has vastly improved since release, especially with certain mods, but is still not as good as the console version unfortunately). Being the first game of the series however, it lacks the polished mechanics some of the later games have.
The atmosphere of Demon Souls is in my opinion not only the best of the whole series, but one of the best atmospheric experiences in video games. This game kind of flew under the radar, but by no means was it a bad game, in fact its an amazing game even by today's standards. Demon Souls was the first game of the Souls Series, and was released on PS3 in 2009. There are 4 Souls games published by FromSoftware 5 if you count Bloodborne, which is very similar to the Souls games but has some distinct differences.
