Painkiller didn't have that dark, satanic appeal of Doom - at least, not for me.
There just was something about going into those rooms with huge, upside-down pentagrams and embossed goat's heads, impaled bodies still convulsing and limbs hanging off meat hooks that Painkiller lacked.
Painkiller's monsters even looked a bit... goofy at times. Granted, it can be scary, but in terms of originality, the monsters in Doom are hard to beat IMO.
I still have Doom, Doom 2 and Ultimate Doom (plus their myriad expansion and mod packs) installed on my wife's notebook. It hasn't weathered that well with time, but occasionally I'll still haul her notebook into the bathroom and play a level or two while I take a dump.
Serious Sam is not scary in the slightest. It is a fun romp and adrenaline-packed run-and-gun shooting, but it lacks that "Doomness". The monsters in Serious Sam can be quite funny, actually (especially those screamers with exploding heads).
Doom II with Doom 3 graphics - can't believe there isn't a mod for that yet. I'd jump on that post-haste.