From the exams I have read or have completed I found that in no particular order. (not sure If i've missed any companies)
Least difficult : Insight, Heffernan, TSSM, Engage, legac
VCAA
Most Difficult : Neap, MAV, Kilbaha, iTute
Insight Heffernan and TSSM I feel can be grouped together, they are probably the very least you want to be doing going into the exam. The exams would replicate the easier side of the VCAA exams. They are good for a starting point but they will get extremely repetitive and if you are doing well they will get boring.
Engage and Legac I felt the exams weren't written that well to even use your time in doing them. Although l used legac for the 2016 statistics sections.a
Neap and MAV would be the best bet (outside actual VCAA exams) in preparing for the exams imo if you are aiming very highly since they tackle the moderate side as well as going slighlty above VCAA level towards and beyond the VCAA separator quesiton level.
Itute and Kilbaha is just there for a bit of fun if you are interested in maths or want to test your "thinking outisde the box" skills since usually the solutions required a non-standard technique. Some of the questions are outside the standard VCAA sylabbus so I wouldn't be too worried if you don't do well on these.