Basically, in the comments, you say a way to counter the previous person's choice of good plant with your own reason. Something a little like this...
Fire Peashooter: I can shoot fireballs and warm frozen plants!
Pepper-Pult: Oh yeah? Well I can warm plant and shoot lobbed fireballs!
Melon-Pult: Well, I have lobbed shots too - they're so strong I have no need to warm plants!
Winter Melon: Well I have all that firepower AND can chill zombies!
Citron: I can one-hit a Gargantuar with my Plant Food ability.
Sunflower: And without me none of you would be here. Case closed.
So let the games BEGIN!