For the purpose of having an index it's clearly Blackcoin that should be added.
Volatile or not, doesn't matter. Blackcoin is top 10 in market cap and has been top 5 in traded volume. Blackcoin is, compared to the other coins, relevant in an index since it is such a large part of the market that you're trying to index.
Quark may very well be older and more stable, but it's a third of the market cap and roughly a hundredth of the traded volume compared to blackcoin which makes it less relevant to add to the index.
If what you're trying to do is just to promote "good" coins, then don't have a poll. Just pick the one you're holding most of.