The early ones are sought after as they are large tabloid size with great cover art. The first one with the dinosaur cover is genuinely scarce. In recent years I have seen copies fetch some eye-watering prices.
I'd also say that 1968 and 1969 are tricky to find, especially in nice flat condition and without that awful horizontal fold across the middle.
The cover art on many of these specials really stands out. Ian Kennedy in particular who did the covers for 1972, 1973, 1975, 1979 through to 1984, 1986 and 1987.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.