Siemens Gigaset are easily the best. Granted not cheap, but then again they are made in Germany with excellent build quality, crystal clear voice quality, allow you to do things like use your own mp3 ringtones, sms to your mobile phone when someone leaves an answering message etc & designed to last you many many years. However just bear in mind that many gigaset DECT models use a separate base station, some like the S795 (which i own) don't. The S810A is an updated version of the S795 and comes highly recommended: http://www.amazon.co.uk/Gigaset-S810A-Twin-Cordless-...
TalkTalk Plus LLU ADSL2+ 18383/1020 kbps Pioneer Kuro 428XD ISF'd iPhone 4S 16gb ThinkPad X220
Can't say which is best, but one to avoid is the BT Freelance we foolishly bought as a "Tesco exclusive" (yes, I know, there's one born every minute ). It's flimsy, fiddly to use, answering device slow to turn off (one must go through the procedure with two button presses) and in all respects it's inferior to the six-year-old Panasonic which it replaced.
I looked around for a good DECT phone and decided on the Panasonic KX-TG6592EM Twin Pack Big Button DECT Phone with 40 minute TAM (Telephone Answering Machine) the answering machine I donít use. I use the Free BT answering machine through Orange. This phone is not posh but highly functional with excellent sound quality.