In my opinion every kind of thread turns into a flamewar. It's like in every new thread being made, the first posters are supposed to make the topicstarter look bad :/
I am not anti-photoshop thread AT ALL. But it seems photoshop threads are the worst kind of thread. They often start off horrible with posts like "I can see the mspaint dicks coming" and "FUCK THESE PHOTOSHOPTHREADS." Then of course there are alot of mspaint dicks and overused memes coming. And thus the thread gets ruined before anyone can even post a quality photoshop. Great job Newgrounds. Photoshopthreads aren't lame, YOU made them lame.
And yet photoshopthreads can be great. When lucky, they are full of great laughs and original content.
Of course there are alot of other horrible threads. Overdone discussions. Vegetarians vs Meateater, Religion against Atheists, etc. And the famous "How did you think of your username" threads. There's too many things to name, it's endless.
I'm not quite sure what the best kind of thread is. Something that's not been done before and where your own opinion counts, like a thread about the news. Not old news obviously.