What kind of alcohol has least carbohydrates?

All distilled spirits have no carbohydrates at all. Vodka, whisky, whatever. Only alcohol, water and aromatic compounds can ascend the spiral in a distillation process.

So a standard alcohol portion (10g ethanol) from a distilled spirit has 0 grams carbohydrate.

Properly made table wine shouldn't conatin carbs either, as all the grape carbs should be fermented. Sometimes substances which aren't recognized in a food are counted towards carbohydrates, that's what a source claims is made with wine.

Then there is sweet dessert wine, there you have 3-4 grams of carbs per 10 grams of ethanol (70-100 ml depending on wine), because it has more sugar. I didn't find information on fortified wine, but as they have more alcohol than sweet dessert wine and probably about as much unfermented sugar, it is about the same or somewhat less.

In beer, expect something like 7-8 g carbohydrates per 200 ml (I am assuming an average ABV of 5%). A big comparison of American beers is given here

I can't help you with liqueurs, they typically are infusions of plants in distilled alcohol, but may have sugar and other stuff added. It depends on recipe.

If you think about it, the idea is to avoid sweet and sugary - so gin, vodka, whisky etc with a diet soda mixer will beat anything else for low carb.

While ethanol is not, strictly speaking, a carbohydrate, I'd consider the effect of the alcohol itself. If your goal is to avoid the weight gain associated with carbohydrates, I'd suggest that the alcohol itself is more significant. While carbs will at least provide energy, alcohol is very poorly metabolized.