I agree with Logan on the bit of painted HL2. It is optiacally one of the most realistic engines, of course, and that is what I always look at, but it is not that awesome.
The texture resolution is bad, I turned it up and it still looked unclear, I find the UT2004 textures better.
The levels also looked a bit to clorful. Imagine all those surroundings in real life, take photos and compare with HL2. You would notice a difference. The sunny places in HL2 looked ok, I think realistic, but where is no sun, they are to colorful and I hate it (in all games).
They are only 2 engines I can think of, that gave me 90% realistic feel, Rebellion's 'Alien versus Predator' Engine and ID's 'Doom3' Engine.
I love AVP and the engine made it AVP in real therms.
Doom 3 is my most hated game ever, but it is the f#ckin best engine+texturing I can think of. Sadly not a good option for INF (not now I guess).
Byside the two I loved Medal of Honor: Alliend Assault's engine a lot, it also gave a very realistic feel.
Not sure about Pacific Assault, while it is part of the MoH tradition with it's realistic feel, I have still many problems with it.
It is very buggy. It runs on low options worse than HL2 on full on my PC

and is buggy. The slodiers run very artificial, they slide over stones. If I fire from cover and clearly see the target 1cm from the covercorner, I always hit the corner.
I find it bad coded. It has some very awesome features like the blurry shaking thing if you fire a heavy recoiled weapon, the model details are huge, but all this eyecandy can´t be used to have 100% clear game (which is the most important in a MP game).
I would wish to have MoH

A for INF3, but this engine has some bad options which let me vote against it
Also it is clearly visible, that it is based on the Q3 engine anyway. It has the 'pak' directories, and if it is not the Doom3 engine those paks belong to Q3, no matter what the Developers say that it is their own engine.
The trees look even worse than MoHAA and very Q3 like.
Oh and Steam sucks hard.
My vote goes to UT2004, as long the SS team make their own more realistic textures. I vote for UT2004, cuz AA:O is my dream of INF. While it is not that realistic looking it still creates a serious military atmosphere.
If SS go for HL2 ok, not bad, but they should limit the steam crap. Updating the steam programm every start (which can take long if the server are overused) is bad enough, but seeing that in INF (mutator terror mutating to steam terror, muahahaha) will drive me nuts.
Pacific Assault could be a good vote, but I think UT2004 can look better and PA is buggy.
Doom3, well better not, even if developing INF takes 2 years, I doubt Doom3's capabilities for a good MP with huge maps.
The real and serious disadvanatge to UT2004 (Doom3 aswell) is, that near everything must be moddeled and skinned. They are no (real) trees, and realistic textures. PA and HL2 has.
Maybe thats why SWAT4 is an engine to think about? Would like to know the opinion of the devs, since SWAT4 demos are out.
@Q3 for INF:
A very updated and pretty much completely new Q3 engine could be ok, MoH

A is the case, but PA requires same power like HL2 or other new engines, lol. And a Q3 looking Q3 engine is definitely a very bad option.