FreeGameDev

(Moderator notice: original post deleted because of spam)

Since this is a forum about open-source games, I will tell you my favorite open-source shooter, which is Xonotic. See: https://www.xonotic.org

Well, Mega Man is my favorite,
it's not free but there are some free
fan made clones, and there is some abandon ware.....
Lorenzo...

Fully Free: FreeDoom (because it is Doom, basically)

Semi-Free: Cube (partly because it is abandoned and so is the same whenever I come back to it)

Source Port: Blood (because it is the peak of 2.5D shooters and I have never quite liked mouse aiming as much; also my favourite overall)

Proprietary; Shogo: Mobile Armor Division (fun setting and story and I actually like the twitchy rough hewn gameplay)

I like Xonotic.

I like the categories of @Technopeasant 's comment, I copy it.

Fully Free: FreeDoom

Semi-Free: Urban Terror, World of Padman

Source Port: FreeDoom

Proprietary; Unreal Tournament 1999

Not to be a pedant (or at least to explain my original intention), but FreeDoom isn't a source port since it is just a WAD file. As a game it is mostly played via PRBoom+ or similar ports because last I checked it was still Boom-tied (though I heard they were trying to make it vanilla compatible in future). I included Blood because although it doesn't have a source code release, there are reverse engineered source ports based on the Build and Duke Nukem 3D source code (most notably BloodGDX and NBlood). This makes these semi-free software, although the Build license isn't properly free (non-commercial use is prohibited; the Duke and Shadow Warrior code are GPL 2) and of course all the Blood game assets and logic are still proprietary.

As for UT99, have you heard of the new OldUnreal 469 Patch? It does wonders for modern system compatibility.

1st place easily Xonotic. I simply love AFPS games and Xonotic is THE best AFPS out there, even above proprietary games. The community is also great. Xonotic is currently an important part of my daily life.

I also used to play OpenArena, which is great too. Freedoom is probably the best single player libre FPS to me -- I have to admit that by gameplay it's a little better than my Anarch :-)))

Talking about open source shooters I would definitely give my preferences to Xonotic like a lot of people here. Talking about non-open source alternatives then it would be Destiny 2. It is about 3 years old but in my opinion it has already become gaming classics. like its gameplay, graphics and soundtracks.

My favorite shooter is Rainbow 6 siege, intense and fast pace of the game really make me addicted to it, but it has a very high learning curve.