Grass is by far the "worst type". No type is "the worst" (unless you count Normal, then, by weaknesses it isn't very good, since it can't hurt rock or ghost types. And the fact Normal doesn't get STAB)
if your only taking Starter's into the option (it says Possibly type), then you guys have a long way to go. there are several great grass types out there.
Ludicolo - Perfect for taking care of fire/ice damage. Rain Dish + Hydro Pump + Rainy Weather can take care of any fire type.
Vileplume - Sleep Powder, Poison Power, and Giga Drain can whisk away life easily, may not be the best type for a fire pokemon, but it's definitly useful.
As for the Starters, they are all great, especially Torterra.
Venusaur - Great Wall. But fire can take it down. If set up correctly, it can take out most ice types.
Meganium - I have nothing for this, since I choose Cyndaquil in Crystal and then lost the game, so I'm not too sure how to deal with it. I hear its a great Wall, though.
Sceptile - Leaf Blade + Overgrow + Scope Lens = Goodbye most types (minus Fire and ice. i've beaten bug with this combo before)
Torterra - Earthquake. It can pretty much take out any opponent. I've taken down the Fire Gym leader before just spamming earthquake on my first platinum playthrough.
Serperior - As stated, once Contrary comes out, your all screwed.