I would say that co.uk has a resale value. .de and .ca are restricted extensions and makes harder find a buyer unless he or she is familiar with the administrative contact stuff or actually lives in Germany or Canada
I don't think there is a great deal of difference, the potential market for each is pretty much restricted to each country, you just need to look at the population or quantity of businesses in each country to order them.
Personally, I only think they are valuable if you are targetting those markets. Otherwise, it is a waste money and time. Because of the restrictions, you cannot own or make use of them. However that being said, the .co.uk is less restrictive and I am using it to host a site targetted towards europe, typically the UK.
Actually, just was wanting to know about from just an investment point of view of those domains for resale.
German market seems to have a considerable financal power. Sedo.com, auction for effektiv.de, current price 1000euros, around $1300. If you look for displayed auctions ending within the next 13 days, it's all about .com, .net and .de, only one or two .co.yu and no .ca. pokermates.us is ending in 2 hours with no bids while rauschgift.de is ending in 3 hours and the current bid is 1100euros, over $1400. kleinkredite.de 2000euros current bid ending in two days. coninvest.de currently 1250euros ending in 3 days, while ecommerces.co.uk has no bids. sternenbilder.de (starsmaker.de in translation I think) currently 220euros, ends not before +3 days, I have seen better domains in English going for $20 here at DP. The highest bid I see on .co.uk is $120 for broche.co.uk auction ending in 2 hours. I see no auctions for .ca. Currently the highes bid at Sedo Auctions is sexkontakt.de for 30000euros. almost $40000 and the auction is not ending before +3 days.