1949 Nickel

Generally speaking, 1948 is slightly scarcer than other dates. That's because most of the coins actually struck in 1948 bore the date 1947, with the distinguishing "maple leaf mark" after the date. In other years there are a few scarcer varieties to look out for, but you'd really need a detailed reference like the Charleton catalogue of Canadian coins to learn the full details of how to spot them. Apart from that, there aren't really any "rare dates" to look for in these series that I can tell.

Here are some examples of post-war scarcer varieties. Canada specialists will probably want to add to or modify this list; I'm working off Krause, here:

1 cent: 1955 strapless, 1965 large-bead-pointy-5.
5 cent: 1947 dot, 1953 strap-far-leaf and strapless-near-leaf, 1964 extra-waterline.
10 cent: 1956 dot-below-date.
25 cent: 1947 dot, 1973 large-bust.