Best customer service, the three times I have had to phone microsoft about my broken 360's. Every time polite, prompt service, quickly resolved, my 360's were couriered from my house to Microsoft and a replacement one back, each time free of charge. Also a nice touch was that they sounded apologetic and like they actually gave a damn.
Worst customer service... in the words of Deep Thought, 'Tricky...I'll have to think about it', although for stupidest, I would have to go for Microsoft again, why do I have to spend 20 - 30 minutes on a phone to cancel Xbox live gold? Why can I not just push a few buttons on xbox live? The customer service itself was fine, it was just ridiculously unneccesary.

The Best Customer Service: Paypal (probably a rather controversial choice). They've consistently helped me, albeit slowly, when I've had some bell end attempting to scam money off me or simply not sending me what I paid for. Ebay also consistently and quickly refund sellers fees from sales which involve Nigerians wanting you to send your item to a train stations. I really can't complain about those two at all, though I know a lot of other people have had massive problems.
The Worst Customer Service: City Link, without a shadow of a doubt. They have to be the worst delivery company in the UK. Not only do they never answer their phone to sort out redeliverys, but I've been cut off while talking to the useless goblin down the other end. The creep literally hung up while I was talking, not to mention the fact they've lost things of mine before. Luckily Ebuyer replaced it and apologised.

Between them, delivery companies and mobile phone companies have to be the worst ever. I remember city link... shudder.
though, it reminds me that one of the best customer service experiences was also a mobile phone company... albeit in Japan. I'd just got a new phone, and I was going a bit crazy trying out all the internet browsing, etc... and then i got a couple of phonecalls in japanese from the company. At that point my japanese wasn't great, so they arranged an interpreter for a 3 way conversation, and called me back at a time of my choosing.
Then they told me i'd run up a ~$200 bill in my first month! :-0
Then they told me i could switch to the unlimited data plan for $25 a month if i wanted, and they'd do it retrospectively, and i could cancel it next month if i wanted. :-o
Incidentally, delivery companies in japan are totally awesome too.. but all the customer service here rocks.
Other best CS experience, maybe Nationwide Bank. My credit card got used for a couple of thousand pounds of online gambling... but i called them up, told them it wasn't me, and they said they'd look into it. They removed those charges, sent me a new card, and that was that.

Wow! There are warhammer and warhammer 40k roleplaying games!
Are they connected to the old WHFRP game??

Worst Customer Experience - overclockers.co.uk
Purchased a new Ninja Uber PC from them and found that the gamer case was locked so I couldn't add additional DVD burners etc. Phoned them up and they said I had lost the key, so not their problem, asked to speak to the manager and was told he didn't want to speak to me. I couldn't believe how badly they treated me (I was polite and just keen to get the problem resolved) but they reacted like I had just murdered their families. Considering I'd spent two grand, I felt pretty damn low....
Best Expereince -
The weird thing is I phoned the Case manufacturers (based in Germany) and they couldn't do enough to help me (even spoke English), they shipped 3 keys for the type of case I had 1st class post to the UK.
I will never ever buy a thing from Overclockers.co.uk ever again.

At first I thought it would be local telco and ISP "Telstra". Fellow Australians, and anyone who has visited here probably knows about the feelings towards them. Normally, they've been rather good to me, though I hate their voice recognition system, which I can only pass with a 'camp' English voice. Don't know why that is, but others are just as bad.
Anyway, getting off track here. I was helping a friend set up their internet connection, wirelessly in the house to a fixed ADSL central point. Telstra, in the country area at the time, the only option.
4 Hours on the phone, up to 11pm before they told me "We have sent you the wrong part", even though they confirmed it was correct early on in the call. Next, the replacement comes quick, but is faulty, a second replacement is sent but then not sent (according to them). Eventually, 3 weeks later, the right part arrives. By then, we have set a wired connection through their building.
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But then I remembered DELL. Their support system involves crackling lines, people who don't quite know what they are doing, people who hang up on you mid-sentence, the usual language accent barrier (both my own and theirs).
What a replacement front panel? You get it in a box 20x the size of the thing and enough Styrofoam to stop the titanic from sinking.
To be fair, they have lifted their game, but now thanks to corporate decisions, we deal with HP.
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Oh, HP. I'm not a fan of the "If you can't say anything nice" type of opinion. But argh! I so badly want to invoke Goodwin and Murphy here.
It's a game of 'Who can get out of the office first' when our area gets the note to call then with an issue.
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Good experiences? *thinks hard*
GOG.com return time on Support contacts.
The Local Cartridge World, who will drive out to our location, even for only collecting a couple of cartridges.
The general store down from work. Food may not be healthy, but it retains that 'community goodwill vibe'.
The nearest Harvey Norman. A Mega Sore that lets me haggle due to their badly done pricing labels, all the others and I can't get away with it.

Oh don't worry. I do so myself while on these systems.
I don't have that 'Aussie' occa type of voice though. I occasionally get people asking me 'When did you move here from 'England'?'.
Though any English person would spot the fact it's not close a mile away. I blame it on too much of "The Bill" as an influence when I was a child. You know, back when the series was about crime and the police...

Ah, yes. The Bill. They still show that from time to time here too... can't believe it's still around.