With so many different methods of shaving on the market, it's hard to know which one is the best use.  Then to further complicate the issue you have a bunch of different brands to choose from.  I will help you solve this dilemma step by step.

Brands:

Personally, I believe the brand of the razor or clipper makes much difference.  I mean what is truly the difference between and three bladed razor from one company than one from another company?

For straight razors, that is a completely different story you definitely want to avoid ones made in Pakistan and China.

Length of Hair:

0-3 Days Since Last Shave

The length of your hair is definitely going to determine the best way to shave your head.  If it's been three days or less since you last shaved your head then a mach 3, straight razor, shavette, will all get the job done equally well provided that you have experience using that particular shaving instrument.  Clippers, will not do well with that length of hair since there isn't enough hair for it to cut.

3-Day To Two Weeks

• If your hair has been growing for 3 days to two weeks then you still have a couple of options to use.  Typically, the mach 3 is useless at this point, unless used in conjunction with a pair of clippers.  The reason being is that your hair at this length will clog the blades of the mach 3 and you will have to keep on rinsing it out.  It will take you forever to shave your head.
• Clippers are a good option at this point because they make it easy to trim your hair short.  Once you have it short with a pair of clippers, it's a simple matter of just following up with a mach 3.  The only disadvantage is that you do have to follow it up with a razor to get down to skin.
• The final option for this length of hair is a straight razor or a shavette.  Either one will cut through your hair with ease.  Hair will not clog the blade since there is only one blade.  The only disadvantage is that there is a learning curve to using them and you are more likely to cut/nick yourself.

Over Two Weeks

If it's been over two weeks since the last time you shave your head or this is your first time shaving your head, then your best option would be to shave it with a pair of clippers first.  After you shave it with a pair of clippers then follow up with your choice of razor to get down to skin.

Irritation/Sensitivity:

If you're skin gets really irritated when you shave your head your best choice might be to just use a pair of clippers.  You won't be able to go down to skin, but at least your skin won't be irritated.

If your skin only gets irritated in one spot, then you might want to try just shaving with the grain in that section.  You won't get as smooth a shave, but that can be a good trade off for avoiding irritation.

Straight razors and shavettes can be a great way to deal with irritation.  These shaving instruments only have one blade, so you are only dragging one blade across your scalp with each pass as opposed to three blades with each pass if you use a mach 3.

In conclusion, the best way to shave your head depends primarily on the length of your hair and how sensitive your skin is.  When you know the answer to those two questions the answer is pretty apparent.