I am not an experienced speaker builder but I would have to answer that depends on what you mean by best. Magnesium cones are measured by Dr Linkwitz to have possibly the lowest distortion of all drivers currently available - certainly of the drivers that he tested. However they have the drawback of having bad resonance problems that need to be tamed in your crossover. So they are certainly not the best from the point of view of crossover design. Also Dr Linkwitz measured the distortion of the accuton drivers to not be as good as the seas magnesium drivers - but some say they have the edge in sounding better - so distortion, while an objective measure, does not necessarily equate to sounding better. My advice for what it is worth is to listen to speakers that contain the drivers you are thinking of using and form your own opinion.
I am currently looking at a design using the DEQX digital correction/crossover so for me crossover design is not an issue. Certainly excel magnesium drivers are high on my list of possible drivers. However what I have decided to do is look at what people who have heard these drivers compared to others have to say and a good source of that are the DIY speaker competition results. Speakers using Excel magnesium drivers have certainly had some good results by winning the DIY Lowa event 2002, 2003 and 2004 However other cone materials have won also - for example I am currently looking at using a Jordan drivers in my design since it won Atlanta 2001 and 2004. A comparison of Excel to Jordan would be nice - any takers?