Codename Eagle - It's a world war 1 fps and it's one of the first fps games with drivable vehicles, including aircraft. The feeling of walking up to an aircraft, getting in and taking off was simply exhilarating at the time.

I'm torn between 2 so I'll mention both of them: XS and Powerslave (aka Exhumed).

I particularly love the setting and level design in Powerslave, sometimes I tend to refer to it as "Serious Sam 0". Also owned an original big box copy of XS, very slow gameplay for an FPS, even doom felt faster.

Not really obscure, but C&C Renegade and Enemy Territory Quake Wars are 2 of the best shooters of all time, but everyone seems to dump on them.

I guess the most obscure FPS I can remember playing was IGI Covert Ops 2 (or something like that). Also Vietcong and Soldier of Fortune 2. Or if you are talking mods, then Monkey Strike (an awesome CS 1.4 mod) was cool running around shooting ar people with bananas and throwing coconuts.

Oh and a mention to Sin Episode 1: Emergence. That I thought was better than half Life 2.

Shadowcaster - fun gem of a game where you had a bunch of different forms you could change into. Each form had different powers and abilities. I remember it being pretty hard in some areas.

I loved that one too.

You just made my day/week. I've been thinking of Chasm: the Rift lately and couldn't remember the name to save my soul. I loved it back in the day but was too young to play it without cheats. Time to try and track it down.

Just looking at the gameplay of some of these old-time shooters gives me the impression the majority of them would not be very fun to play anymore. Awkward/outdated mechanics and extremely dated level design seem to be the main problem.

I've finished that game a long time ago, and mainly I recall how I tried to trick enemies to get stuck on wall corners so that I could hit them without them hitting me. A bit similar thing I did a lot in The Elder Scrolls: Arena.

To me it felt... ok, but just ok. I recall i liked its music somewhat as it fully supported Roland Sound Canvas (GS).

Shogo: Mobile Armor Division I liked the anime influence and style.

Breed.
https://www.youtube.com/watch?v=4OVtZaYJ8ak

It was slightly broken game with redeeming features. One of the few games at the time to let you put vehicles inside other vehicles and freely move around inside them. For example driving a buggy into a drop ship then flying off.

You could fly the dropship from a mother ship in space down to the planet or vice versa.

https://www.youtube.com/watch?v=EZNGWxMoI68

Deadly Dozen 2 Pacific Theater

AMAZING GAME

the first one sucks beyond all known reaches of suck.

A bit late, but that's incorrect.
The first Xenus is Boiling Point, the second is White Gold.
Along with Precursors an interesting trio of games, conceptually at least.
I'll never forget one of the patch notes for Boiling Point.

Vivisector: Beast Within, by 1C Company, same guys from Cryostasis.

Island of Dr Moreau + guns, basically is a game about shooting furries. :P

It starts slow and awful, but I'm glad I stick with it through the end, the levels get crazy good.
Wish it was available digitally. :/