I would agree with Trade_BTC that silver and gold are probably a better choice than brass.

However, what seems more important for Denarium coins to become a collector's dream in the future is the number of coins in each edition.
I don't think anybody will seriously collect Denariums coins if there are dozens of completely different coins with each just 100 coins in circulation. But if there is a themed coin with at least 10.000 coins in each edition and a new theme every year, that will become the coin that will be famous in the future.

So far I have been buying only the 1/10th and 1/100 brass coins and two 1 BTC Silver bull coins. The first ones as gifts and to gain collector's value, the latter ones will be for my two children when they are grown up.