Based on my personal experience and nothing else:
By far the best online supplier I've ever used has to be Kustom PC's. Bunch of young Scottish guys, you can reach them on the phone and they're very friendly and helpful. I bought a case in a sale for £20.00, the PSU packed up, I phoned them and they sent a replacement PSU the following day. Now that is service
I've never had a problem with them and their orders arrive quickly.
Unfortunately, their stock range is limited but they've got some great modding stuff.
Second up, overclock.co.uk are good, you can speak to them on the phone as well, very helpful, never had a prob with them, they have integrity.
And that's about as much as I'd recommend.
Having said that, mostly I buy from overclockers.co.uk as their prices are good, they have a wide range of stock and 90% of the time they're OK.
However, they have no e-mail service, you can only contact them on the phone or by fax and their phone is always engaged. Sending a fax will get you a response in 48 hours though, generally.
And I'm still smarting from two things:
1) A 5.1 speaker system I ordered was DOA. It cost me £9.36 to return it, I never did get that money back. Which made it not such a bargain after all.
2) I ordered a Hercules 9800 Pro, they sent me a Powercolour 9800 Pro. Now, admittedly, the Powercolour cost more than the Hercules, but the Hercules had been reduced in price and that's what attracted me to it. They should have e-mailed me offering a choice, not just sent the Powercolour card. I think the whole thing was a con to shift their Powercolour cards.
However, the card they sent me is still going strong, some 6 or 8 months down the line.
I've used most suppliers, including Dabs, Komplett, Tekheads, Scan and CCL. Had a minor problem with Tekheads, the rest are OK (so far).
Actually CCL is good, but limited stock.
I've never used Aria but I have heard a few horror stories about them.
Stores to avoid (imo): Redstore and E-Buyer.
PS/Edit: One last mention: Chillblast, almost forgot about them, They're great
But expensive
