Always teach the vowels first. By starting with the vowels, statistics are in your favor. There are only five vowels vs. twenty-one consonants. At least one vowel appears in every word in the English language. Quite simply, once a little learner masters his/her vowels, he can already begin to recognize sounds within every word.
Cast off tradition. Although most pre-readers have learned the words and tune to the popular ABC song long before they actually know what an A, B, or C really is, they should not be encouraged to learn the formation or sound of the letter in that same order. Experience has shown that a child who learns the sounds of his letters in alphabetical ORDER has a harder time recognizing them out of order...as in the case of a REAL WORD. You will often times see these children singing the ABC song out loud or in their head in order to remember the sound or name of a particular letter. The "habit" that was created as a toddler of seeing and singing the letters in a particular order is hard to break. But if the formation and sound of the letters are taught APART from the song and the song order, the child's reasoning skills will categorize the song as one concept and the sound/formation as a completely different concept. Therefore, it is much easier for him to learn to truly READ the letter.
As a final thought...Although I introduce both the capital and small version of each letter together, I emphasize the small formation the most. Let's face it, the ratio of small letters to large letters in a book is quite substantial. Typically only the first letter of each sentence or an occasional name is ever capitalized. When choosing educational toys and games, you can always spot a QUALITY item if it was created with small letters vs. capitals. Not only for the purpose of reading, but also for manufacturing costs...it is cheaper for a manufacturer to produce a toy with straight lines (as in the case of most capitals) than it is to produce a toy with both straight and curved lines (like small letters). This is why most QUALITY, small letter toys (puzzles, push-button games, blocks, etc.) are more expensive than their capital letter counterparts.
Although every child is uniquely wired with his/her own unique way of learning, teaching most little learners to read the ABCs is as simple as A-B-C.