Fire/Poison
Ice/Rock and Ice/Steel both seem to be boring to me. So much can be done with Fire/Poison, and I think it will be a fun type to play with further down. I'd also like to point out quickly that while Ice/Steel is at first apparent as the perfect Dragon Slayer it has a crippling 4X fire weakness, and nearly every viable dragon is capable of running a fire type move. Unless this mon has either ridiculous levels of bulk, an ability to combat this, or ice shard, it's far from a good dragon slayer. Ice/Steel would be my second choice.