Silicone ones are what they often used originally, but IIRC that's mainly because they give longer life - they're not exorbitantly expensive in a per-unit-price sense, but when you have to buy a whole pack of them at once it's pretty steep so I don't see the point. For us homebrewers who can keep a better eye on them, the cheaper ones are fine.
All my kegs have seen many batches of beer and I haven't replaced the o-rings since the initial reconditioning - and I haven't seen any signs of wear. I guess you can replace every time if you want, but that seems pretty excessive considering that normally the post o-rings literally only see any kind of stress twice - once when you put the disconnects on, and once when you take them off.
