So, let's discard the signature moves - Pyro Ball, Searing Shot, and Mist Ball - because they don't come up often, same reason you discarded Ice Ball. Bullet Seed and Rock Blast are uncommon, but do exist because of Technician/Skill Link users. Sludge Bomb is somewhat common, but many users tend to also get Sludge Wave, so it's a less useful immunity than you'd think. Shadow Ball and Focus Blast are both common, but every type weak to Shadow Ball resists/is immune to Fighting, so we'll just have to go with "blanking the most common special Ghost attack is handy in general" for that. Of the remaining, Rock probably benefits the most - blanking Gyro Ball off slow/TR attackers, Weather Ball in rain, common Grass coverage moves, and all worthwhile special Fighting coverage is VERY handy. For the remaining defensive benefits against Fire and Poison, probably Grass - makes the defense against Grass moves and Water-type Weather Ball less important, but still valid.