Now that I've finally acquired The Reaper title I feel like I have good enough understanding about which monsters I like the best and which ones I don't. The worst one in my opinion was probably Araxxor/Araxxi. The fight mechanics are simply too annoying and the fight takes too long to secure a kill. Not to mention the antifarm measures that make them even harder to kill once you finally get one down. It took me hours to secure just one kill and ever since I really haven't felt like going back to try again. Considering that many of the best drops aren't even tradeable, instead you have to get many pieces (which means even more kills) to finally get something to show for it. Uggh!
I never really liked strykewyrms either, mainly due to the fact that you have to stomp them to start the fight and the fact that they do that digging move which makes them annoying to kill. Airuts are a bit annoying due to their high defence. Kalphite King is not my favourite either, mainly due to the mechanics that really punish for mistakes at wrong times. Like the one hit kill moves and the mage phase balls.
Araxxor and Kalphite King could be better if I had the motivation and time to learn fighting them properly. But as it stands now, I really have no much interest to kill them. Since I've killed like over 5k abyssal demons for daily slayer tasks, I don't really like them either. The teleporting is annoying as it often cancels good abilities (like destroy/rapid fire etc.).
I guess I could figure many more annoying monsters but those came on the top of my mind.
When it comes to my favourite monsters I really like all of the GWD bosses (including Nex). Though ever since Nex got buffed I haven't managed to solo her anymore. I liked Nex before that the best.
I actually found the fight way too easy. First death was due to attempting the mage version when I was melee, second because I yoloed and berserked on p4 and died with her at 6k.
The absolute only thing make that fight difficult is RS tick based combat system. There is no hard RS boss, mechanic wise. It's dealing with their mechanics in RS tick based, buggy, and often weird system that makes fights difficult and annoying. Just like Estoc said.
Personally I hate mole. Bugger is annoying.