One of the many things I love about the S.T.A.L.K.E.R. series is that there are no "best weapons." It is all about personal preference. While the stats on a weapon may be high, you might prefer the "feel" of a weapon with lower stats.

My usual loadout in CoP is the unique variant of the SGI-5k with an attached grenade launcher, often switching between different scopes. I like to keep the silencer off most of the time, because I like the sound :)
My secondary weapon is the Eliminator Shotgun with the 1.4x SUSAT scope.

On my current playthrough though, I am having a lot of fun using the Vintar BC and the Hunting Shotgun. It is a great weapon combo for silent kills at mid to long distance, and a quick two-shot epic kaboom at close range.

My favorite is the Vintar BC- often let's me pick off entire groups before they can get close enough to reliably return fire. I tend to pair it with the Tunder 5.45 (aka Storm in CoP) for use as a short-range weapon against mutants.

I like the AK/S-74 and the Obokan assault rifles, playing with anything different in the zone just feels wrong. =P
I also like the Viper especially if you can have the rechambered one you can switch between a normal and rechambered and never run out of ammo.
For shotguns I like the double barreled hunting shotgun the most, too bad you can't hunt ducks in the swamps and have a pseudodog to fetch them. =D

Weapons I didn't like: that british assault rifle (don't know what it's called), the double barreled sawnoff (well it can be fun in some booh I got ya cases).
The Vintar is good but I find that the game becomes too easy with that weapon and that is no fun.

I was also thinking it would perhaps be more authentic and fun if early guns would be really old soviet and german ww2 vintage like Mosin Nagants, Spagins, MP-40's and Lugers.

I like those too. If you're decent at headshots you can make it through most of the game with them.

I tend to keep to the an-94 with the vintorez; an-94 for its 2-shot burst and the vintorez for a smg/sniper hybrid.

Aside from the guns, do you guys switch to AP or do you usually keep to the common rounds?

Anything that reminds me of an AR-15, as well as a Colt .45.

Vintar Bc and Vodka. haha

I have many loadouts. I think that every weapon is useful in specific situation. But still i have my checked loadouts for every game:

ShoC:
Usually i was going out with SPAS12 (SPSA14), Sniper ZM LR300 (TRS301), and IMI Desert Eagle. As a sniper rifle i usually used SVD, but since it appears late in game i dont use it much.

CS:
I really loved silenced I85 (IL86) upgraded for maximum accuracy and shooting 5.45x39.