I have examined the effects of every weapon on every enemy in R&C1. The Pyrocitor is very effective on Aridia as you battle the Sand Sharks to defend Skidd. There is manages to take out the 3 hit spawners in a single sweep and is good for crowd control. It can also be useful for battling the Squishy things in certain areas of the Blarg Research Station and the bomb droid things. So yea it's a handy weapon you'll want in your quick select.
But the Lava Gun just had such a cool design and function. The streams were beautiful as you spun around and targeted your foes. Not only that, but it's even more effective than the Pyroictor in the games it's featured in because it can be used at greater ranges, it protects the player from all directions against smaller enemies, and it can be flicked to even deal damage to flying enemies by timing the firing and double jumping well. I loved the things you could do with this weapon when you spent enough time using it, and I think it's an easy #1 for me.