Due to the nature of trench warfare, some form of poison gasses would have been really useful in the trenches. In the trenches the gasses really would’ve been really useful with the limited mobility of the soldiers and would resulted in maximum enemy casualty. So what types of poison gasses were used in WW1?
As noted in another answer, the Germans introduced chlorine gas early in 1915, the French countered with phosgene gas later in the year, and the Germans "topped" this with mustard gas in 1917.
Gas was used mainly for shock value. The German apparently gained an advantage with "first use" in the Second Battle of Ypres. But gas is hard to store and transport, and the Germans soon lost their initial advantage when they ran out of gas.
And in terms of "killing ability" gas was less effective than other forms of weaponry, because it needed "ideal" (e.g. weather) conditions to be fatal, and because antidotes such as gas masks were easily developed. That's why unlike other World War I innovations such as the "tank," gas never really "caught on" as a weapon.
A number of poison gasses were used in WW1. The first use is by the Germany at the battle of Second battle of Ypres. But, by 1915 the French managed and even more powerful gas called Phosgene. Phosgene took 24 hours to manifest itself, chlorine had a greenish cloud and a very strong ouder. But the most effective chemical weapon was mustard gas which, despite not being particularly fatal, was great for disabling the enemy or polluting the battle field. It was used in the third battle of Ypres by the Germans.