Hmm, I haven't played Polarium, I thought that would be the cheapest of the three.
Zoo Keeper is the only of the three I have played, it will keep you going for a while, it's addictive as hell and if you have a buddy with a DS the multiplayer mode will provide a bit of fun. My G/F who doesn't play games that much plays it religiously, if that's any indicator of how enjoyable the game is..
I would go with Polarium, then Zoo Keeper, than Mr. Driller.
I think the RRP for Polarium should be around the $50 mark, I know that's what it sells for here in Aus. (places like Target, K-Mart).