All coins pre-1921 are silver. Be aware that during WWII, there were two halves of France: the Free French (using the same designs as pre-war France), and the Axis-aligned Vichy state. The easiest way to tell them apart is the motto: Free France used the traditional "Liberte Egalite Fraternite", while Vichy used "Travail Familie Patrie".
Dates to look for:
50 centimes:. Of the silver ones, 1905, 1903 and 1911 are the scarcest. The 1947 is the scarcest of the early aluminium-bronze types. There's also the 1962 "4 folds in collar" variety is also pricey.
1 franc: 1914C, 1903, 1906 and 1907 are the scarcest silvers. In al-bronze, 1920 and 1935 are dates to watch for. In aluminium, the Free French 1943 is one of the great rarities of modern French coinage. On the Vichy side, the zinc 1943 is also quite scarce.
2 francs: Anything pre-1914 is worth looking out for. Al-bronze: 1920, 1926, 1927, 1931 and 1935 are the dates to watch for. In aluminium, the Free French 1944B appears to be scarce - no value is recorded in Krause, and the mintage is quite low.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis