I've been using Tyrann's software engine, which supports bsp2 and helps me pretend like its 1996. 

 

My name is onetruepurple and I use fitzquake5.exe most of the time. 

My name is mfx, and I use QuakeSpasm 0.85.9.
2nd is fitz 0.85.
3rd choice is the engine Ijed mentioned. 

...unless a release calls for a specific engine to run. 

1st: unreleased engine. based on joequake with fitz,bjp,etc parts and influences.

2nd: quakespasm, usually freshly compiled from svn. 

which is DirectQ 1.9.0 

I used fitzquake as a close second 

My name is ScampieSUXCOCK and I use Quakespasm. 

And I use FitzMkV, or RMQE 

I remember an earlier version I tried did not animate the sky textures correctly on func_walls, which was something I had done in Castle of Oblivion. So I stayed with GLQuake for a while, but I think with 081 I switched for good. 

for a long time i've been using aguirres enhanced engine

and now i use fitz85 (modified to prevent that infamous "whats that name of willem map" bug) 

Lately I have been using Fitz mark V with bsp2.
And Tyranns engine, as second engine. 

 

This would have been a great discussion thread!

Anyway I use Fitzquake latest SVN version (which is basically 0.85 with 2-3 half-baked new features) 

I vacillate between fitz .85 and the RMQ engine. 

I AM SCAMPIE I USE QUAKESPASM

Mostly because it's like Fitzquake, but also plays .ogg files from /music/ 

My name is Tronyn and I tend to use Jack Danieeaas I mean FitzQuake but with a strong secondary reliance on both DarkPlaces and RMQengine 

I am JPL and I have two reference engines (aforementioned)

I personally use both of them depending on the map. No need to say FitzQuake is the reference engine I (have been used to) map with. but I always give a try to aguirRe GLQuake, because it has a different look ingame (i.e light and fog density are not rendered the same)..

Well... Fitzquake is my preferred engine in anyway ;) 

Also, FitzQuake 0.85, because it comes with no bullshit and I haven't found a reason to use anything else yet. 

Some would say 'bloated'... 

and I used FitzQuake0.85 all of the time 

Is nice and rounded engine, but I can't use it to play my maps.

I use it when I record demos for others. 

http://www.youtube.com/watch?v=rWy6MNLuZLY