If you want to bring in SCE's, then none of them are useless. Well, they all have their uses in various situations. All will wreak havoc in SP mode just because of how fast the unlimited ammo comes out of the shield. The Fire and Plasma Shield ammo can make for good eye candy when inside a crate that breaks, and the Laser Shield ammo has a variety of applications particularly in Treasure levels.

Considering the lack of limitations set forth by the topic name, I think most people would agree each type of ammo can be useful in at least one situation.

But with some rewording, another argument can be raised. I believe that the Fire Shield ammo is the least useful because not only does it give a general idea of where its source is for players below, but it only inflicts one heart damage. Combining that with a limited range, you're almost always better off using a different type of ammo than the default of the Fire Shield.