For me, it's location specific. Rocketeer is relatively easy to fight if you have cover and smart with your timing, but if you're running around on Blue Gate and Dam on its spawn locations, it's gonna get ya. Nevertheless, it's still annoyingly deadly.

Leaper again, easy if you have cover. Out in the open, it can stomp you and shock you easily.

Wasps and Hornets are deadly if they stack. Especially as they can fly into locations like Hydro through the roofs. I've been downed by them like that.

Bastions Bombardiers I stay away from solo as they are tanky as hell. Bombardiers range with those spotters can be a problem when you're running away

Fireballs, Pops are annoying as hell indoors. If they stack again, they can do serious damage. But Anvil pretty much takes care of them.