The Shivermist Buster (5* freezing vaporizer) is one of the most common, and most useful (in my opinion). You can use it to stop gun puppies from turning, you can use it in Citadel and against Vanaduke, and dealing with things that don't move makes things much easier. It usually can put out lit oilers, but it's not a guarantee since they can be ontop their flaming oil slick and just keep getting re-lit. Also since you have CTR med on it that's a great choice.
Ash of Agni (5* Fiery Vaporizer) is great for JK runs since everything dies so much faster. Can be used to de-frost some freeze typed enemies and the effect lasts pretty long.
The Toxic Vaporizer would be a great choice, but the effect lasts too short that you pretty much need to be spamming the bomb to really make use of the poison. Pretty decent against JK to stop him from healing if you can manage to get a charge done without getting hit.
Your Glacius vs Combuster really depends on the bomb (not always though), if you are getting a Ash of Agni then a Glacius would be a good choice since you can use the charge attack for Citadel and use the Ash for Frozen themed areas.