Best ad of all time: The Maxell one with the guy holding up the words to "The Isrealites" and getting them all wrong. Will see if I can dig up a clip.
Worst ad of all time: Plenty of competion here, what with all the Finance/Insurance/Micheal Winner/"I tripped on a paper clip, now I am going to sue my employer" ads, I think the Micheal Winner (anything with him in) ads win though..
The best recent ads have been the Peter Kay John Smiths ones.
Two sites with interesting (if small) libraries of classic ads:
http://www.absolutelyandy.com/tvadverts/index.htm &
http://www.uktvadverts.com
Edit: Found the Maxell Ad (thanks to Yesman's site) http://www.visit4info.com/details.cfm?adid=14902