Broadhead choice will always be a debated subject because there are so many choices. The main starting point is to determine what animal you are going to be hunting. This is important because of the simple fact that mechanicals do not penetrate as deep as most fixed blade heads. I want whatever I shoot to have 2 holes that they are bleeding out from.
For whitetails, I use NAP Killzones and I have never had an issue with penetration...they blow right through deer. I choose to use them because they leave a nasty 2" entry/exit hole and they are mechanicals so they fly extremely well.
For elk, I use a 3-blade fixed broadhead. Elk are much thicker than whitetails so greater penetration is needed. When you start shooting fixed blades, much more attention to detail is required in your set-up if you want to get really good groups at long distances. You are essentially adding fletching to the front of your arrow so it is necessary to paper tune to make sure that your arrows are coming out of the bow straight. Also, adding larger fletching or using helical/offset fletching will increase the resistance to the back of the arrow thus making it more stable in flight.
Another consideration is making sure that your broadhead's ferrule is straight. This is accomplished by spin testing your arrows with the broadhead. A crooked broadhead will add resistance to the front of the arrow and steer it away from the others.
If you want to get great flight out of fixed blade heads, put in the work to make them fly well. Montecs are great broadheads. I stick to shooting a 3-blade fixed over 2 or 4 because they are a happy medium. 2-blade heads do not cut as much as I prefer and 4 blades simply do not penetrate as well for the most part. Keep in mind the surface area of the blades...the greater the surface area, the greater fletching stabilization is required and the more resistance you are going to have at the front of your arrow.