If you're a die hard purist, go for the originals (the source code is available as well), revolutions is more or less gay. HD verisons are an okay substitute and burrows some aesthetics from the XBOX version, but something about secrets and no harpees holding headless kamikazes if I recall.
always stick with
SERIOUS
The problem in the original games is that with normal is that it's too easy, the problem with hard is that the ammo is not adjusted finely to match with the difficulty. Serious in all games are fine enough to play with the autosaves (it usually autosaves before each encouner) but not frustrating enough to savescum through.
Serious's big difference is double ammo capacity and the amount of fucking enemies spawned.
There's Mental but it's usually just Hard but the enemies are invisible, not really a challenge but gimmick. Yeah you do take double the damage, BUT only in Serious Sam 2 do enemies receive increased health (25%, not that bad). Serious Sam is iconic in how many enemies it throws at you and the manic effect of being almost overwhelmed.
Co-op also makes the game more challenging and introduces way more enemies as well.