My favourite is probably the lasers, if that counts
Though I don't overuse them.
Least favourite, if we mean one that I find hardest to deal with, would be those slow moving disk creatures that shoot a random spray of bullets. They're quite challenging because of the rate at which they fire, you have to find cover near them, dash for it, and sometimes, navigate through the moving bullet spray. Put another enemy or hazard nearby, and we have quite the challenge
Most of the enemies have a way of using them that makes them well balanced. You just have to think about how Nifflas would use them. However, what I hate the most is when designers give you an enemy with fast, random projectiles, when you don't have the run ability. You're completely at the mercy of the enemy's fire and it's dependant on luck whether you get past, most of the time.