It's All About The Right Ingredients
A Mojito is traditionally made of five ingredients: white rum, sugar (traditionally sugar cane juice), lime, sparkling water and mint.
Also known as "silver" or "light" rum; it is a dry, light-bodied rum, light in color and lightly sweet in flavor. It is produced from sugarcane juice and molasses, and normally has a short aging period. The majority of Light Rum comes out of Puerto Rico. Their milder flavor makes them popular for use in mixed-drinks, as opposed to drinking it straight.
Unlike other various rums which are aged in charred oak casks, light rum is simply aged in plain oak casks or stainless steel tanks, so it remains clear. Light rums are also sometimes filtered after aging to remove any color.
Some producers of light rum are:
10 Cane, Bacardi, Cruzan, Havana Club, Malibu, Mount Gay, Ron Don Q
The Sugar Debate
There is also some debate on whether simple syrup or sugar is better: I like them both but prefer organic cane sugar. The advantage to the sugar is when you pour it onto the lime and mint before muddling, it's natural abrasiveness helps to bring out the flavors and the texture adds something to the drink. The advantage to the simple syrup is that you can make a quantity of it to have on hand and it is easy to store and measure to serve.
(You can also infuse your simple syrup with a complimentary flavor or with mint if you are so inclined.)
Lime Juice or Fresh Limes
Fresh limes will always provide a better flavor and the oil from the rinds is essential in obtaining that signature "fresh" taste and full aroma of a Mojito. After going through the trouble of finding fresh mint, don't skimp on the limes. It's one of the predominant flavors and will really make a huge different in taste. Sometimes, however, they are just not available. So, in a pinch, lime juice from a bottle will do.
Sparkling Water / Club Soda
Use a high quality bottled mineral water or club soda in place of seltzer. Remember, garbage in, garbage out. You're making a mixed drink with several ingredients so the overall quality will be determined by what you are starting with.
Cuban Mint / Yerba Buena (Mentha x villosa)
While recipes call for any available variety of spearmint, the real mojito can only be made with the true mojito mint. This mint was impossible to get in North America but thanks to a Toronto mojito enthusiast who visited Cuba in 2006, we now have access to the authentic plant from Cuba. It is clearly different from most other mints -- its scent and flavour are agreeably mild and warm, not pungent nor overly sweet like other mints. In a perhaps typically Cuban understated way its warm embrace lingers until you realize you want more. Like all mints it is easy to grow and will happily provide more than enough fresh sprigs for your mojitos.