i have played a bit of first encounter, played several times 2nd encounter, played SS3 and i got the DLC this sale and i played a bit, and played serious sam 2 too
From the old games, 2nd encounter is the best one... GREAT GAME... the coop is super fun
and if you like crazy, bizarre things in your games, its great.
Like running a bit off the map to a spot with an armor. When you pick it up a message shows up "YOU FOUND A SECRET ARMOR" and as soon as you read that a rock comes down from the mountain and, if it is your first time getting it, crush you and take all your recently picked up armor. Or rooms with weird physics in witch the room spins, or everyone bounces. I loved it
SS3 its a bit more mature, but it still has lots of enemies, some big ones, crazy weapons and coop all the way from the start to the end and funny dialogues and parts. And the graphics are surprisingly good for such an "unknown" game
you can go bad... just buy it, i recommend it