First use a flashlight and check the seat carefully. Some are square drive and some are hex drive.

If it is square drive, make sure your square drive tool is long enough to reach all the way through the seat. If its only making a partway contact it could be causing the seat to bind.

I am assuming your using a square drive socket on a ratchet. If so, insert the square drive fully, then hold the ratchet by the head with the ratchet handle pointing up. Turn the ratchet until you have full firm contact with no slack in the drive, then rap the handle firmly with another tool. (I use a 12" crescent wrench to rap the handle). Generally it only takes about two or three good raps and the seat will free up so you can turn it.

DO NOT USE ANY PENETRATING OILS, remember that is potable water flowing through that valve and you do not want to risk getting chemicals into your potable water.

Once you have the seat out, take it to a True Value or Ace Hardware store and they will have replacements.

When selecting a replacement you will note that they have Brass, Monel Metal & Stainless Steel seats.

If your seat has what looks like a chip on the bottom side of the face, that is generally caused by water quality. I would strongly advise you to pay a bit more and get the Stainless Steel seat.

Also, after you install the seat, Make sure the stem is turned to the full open position before you screw it back into the faucet. If the stem happened to be in the closed position, as you make the final turns to tighten the stem body into the faucet it would push your new washer into the face of the washer and instantly distort and ruin your new washer.

I have a simalar situation with my shower diverted valve. The seat head stripped and its become a round hole. Can I carefully drill it so I don't damage the threads that a new seat threads into? Need help

Here is a tip that has worked for me repeatedly... use a flat head screwdriver that has a tapered blade (most do) use one where the tip will go partially into the hole of the seat. Tap the end of the screwdriver with a hammer a couple of times. This will do two things - it will cause the blade of the screwdriver to cut into the seat to grip it and it will also cause the seat to break loose. Use channel locks on the screwdriver to unscrew the seat.

I've maintained several hundred apartment units and this never failed me.

The key to getting them out is correct fit from the start. Square and hex both come in straight and tapered holes. Tapered holes come in two different degree of taper so it require's at least half dozen wrench set's to be ready for all of them. Ask a pro if he has means of removing striped seats. If he tells you the seat can not be removed and you will have to replace the facuet,you have noting to lose trying to remove seat. You will never drill it out without damaging threads so don't drill. Using what would have been correct wrench set,.go up one size and drive it in. Once wrench is imbedded in seat,use vice grips to turn as you continue hammering. The secret is in knowing the instant seat break's loose and stop hammering but keep turning,else you ruin threads. If you are unable to find correct wrench set,buy an ease out at auto supply store. Insert ease out then tap while slowly turning untile ease out take's a solid grip,unscrew set without tapping.

I used an EZ out but, to protect the back side of the plumbing, I did not tap it in with a hammer as you would normaly do. I gently pushed the EZ out in by hand and held hand force to it as I gently turned the EZ out counterclockwise with a wrench until the EZ out obviously bit into the inside of the seat. I continued to turn gently until the rusted valve seat "popped" indicating that it was now free to thread out of the fitting. An EZ out with a short, blunt end is best because the tip of a longer one might damage back end plumbing. It is important to be gentle when working with plumbing. Lars