Which one would you buy and why?

I can't decide really...

Which is most reliable in field conditions?

You're comparing apples to oranges to pineapples!

AR-50 is a great but(what I would call.) heavy "Bench" type of gun. IMHO.

Serbu is a lighter, therefore harder kicking, "Field" type of gun. Hey! I own one I know! It's a great shooter, but I wouldn't want to be putting lots of rounds downrange each day with it!

Grizzly is kind of between the two, but closer to the Serbu than the AR-50. Also a nice gun.

What I'm trying to say is that You need to know "What" You want the gun to do!?! Is it a target gun? Bench shooter? Are You going to want to walk around with it or set it up and shoot it from a bench? Is cost the main factor or is accuracy Your prime goal?

I own both the Serbu BFG-50 and a State Arms Knob Creek 2000. One of each Type of gun, (IE: Serbu is #25 and portable, State Arms is #41 1/2 and a bench baby.) I can shoot the State Arms all day, as it is almost like shooting My AR's, the Serbu is like a Mag shotgun. Figure out what You want to "Do" with the gun, then drop Me a line if You need more info.

Tall Shadow

Barret M99, low cost and super accurate.

Beyond the shadow of a doubt.
Barrett M-99...........i have 3
Eric the ammoman

Before you limit yourself to those choices, take a look at the AMAC 5100. Best .50 I've ever fired. Cost about $3300 but come complete with numbered rings and a bi-pod. Only other thing you need is glass. Just about every thing on the weapon is milled and the gov't cost was near $8000 in the 80's. Guarenteed to shoot under 1 moa out to 1000  yards w/match ammo. They weigh 36 pounds and the recoil is similar to a 12 ga slug. There are several of these guns that have over 10,000 rounds fired through them, and are still going strong, but a little is left to desire as far as the group size goes after that many rounds are sent down range.  

Of the three listed the serbu