I've done quite a few Spec Maths ones and this is my opinion of them:
VCAA - This is the benchmark - I can finish them in time, their questions are quite hard, but not long-winded, like they trip you up on true difficulty rather than time or nasty tricks
Insight - They're easier than VCAA but they do sometimes have a few good questions - they're not that bad!
MAV - Similar opinion to insight, there are some good questions, but overall, the whole paper is easy
NEAP - A little bit harder, some more long winded questions - some questions that are what you might call "dodgy" - questions not like VCAA ones - but good overall
Heffernan - Probably the best in my opinion, they reflect VCAA quite well and have good questions - good mix of hard and easy questions like VCAA
Kilbaha - They're hard, probably a little bit too hard, their questions aren't so much that you need skills to solve them, but you need to work efficiently
iTute - They are what you call dodgy
TSFX - They are what you call dodgy
TSSM - I've heard them to be hard, but I'm not too sure