>>12427260>>12427427>>12427597If you look at it purely by the numbers, grass is the worst, in that it has the most weaknesses and is weak against the most types.
However, I'd say that grass is far from the worst type in practice. It's the only type that resists water besides dragon and water, and the only type that resists electric besides dragon and electric. If we look at it's weaknesses, it's weak to poison, flying, fire, ice, and bug. Poison type moves are extremely rare, due to it being the worst offensive typing. Flying type moves are also uncommon.
It's not that bad offensively either, it's one of the only two types strong against water, which gives a useful niche at the least. More than you can say for poison or normal.