I was wondering what some of the top favorites are. Anything that is older than about 2006, which is when I would basically consider the genre to have flopped due to consolers.

Mine would definitely be Tom Clancy's Rainbow Six: Rogue Spear and Raven Shield.

Not quite what I would call "Old-school", but to each his own :) My favorite must be Doom II. That game was brilliant...

Quake 2

There is just something special about that game

BTW Old skool to me is pre 1999

Hexen II

I loved the Rainbow Six games. And Delta Force. And Doom.

System Shock.

I love my old arcade-style shooters of which my favorites are:
Blake Stone, Doom 2, Duke 3D, UT '99

Modern shooters may be fun for a little while, but they just can't keep my attention for long.

A tossup between Unreal and Doom 1+2. And System Shock 2, I suppose, but it's only a "kinda" FPS.

EDIT: Quake needs mentioned too. Awesome atmosphere and even more refined design (in some ways)

My favorites are Strife and Shadow Warrior.

Heh, yeah, you'd expect me to say System Shock, but the fact is that both were quite sucky at being FPS games. My answer's Blood and Unreal, pre 2000.

Was never really into FPS but I remember playing Doom as a young manling. Today it looks like cardboard men shooting staples at each other but back in the 90s that was the Crysis Engine!

I can't decide between Hexen 1, Heretic and Shogo.

If we're speaking about old-school gameplay-wise, it's probably Painkiller.
If we speak about real *old* FPS games, I can't decide between Rise of the Triad and Blood.

Ahh, those are both really good as well.

Painkiller's not oldschool! Action-packed shooting just isn't enough to constitute as such.