Argos - stupid mare didn't press button to say my order was available, so I stood there waiting for ten fucking minutes.
Also Morrisons, not sure if staff in all branches are as hopeless as the ones in mine, but 90% of them are incapable of doing basic tasks assigned to them (like a checkout woman, unable to operate a checkout). They are surly, rude scum.
I like to get my own back in a small, petty way wherever possible. With Argos, just go to their website and reserve lots of bulky items (furniture or exercise equipment are great), then put in a random email addess. Mildly amusing, especially if you make sure the items are not in stock at the branch you pick, so they have to go to the effort of transferring it around, wasting their time and money.
When I get shocking service in a supermarket, I just pick a day when I'm at a loose end, then go and fill a trolley up with as many items as I can cram in. Always put crushable items at the bottom, and heavy items on top, and make sure to chuck in some expensive chilled or frozen items that will spoil when you abandon the trolley and walk off. Make sure you use an unlocked trolley of course, you don't want them getting your pound!
While these petty acts of "revenge" won't make them improve their service, at least you get the satisfaction of hitting their profits.