How should I say, "The most tragic" or "the tragicest"? According to the rule, the second one is more preferable, but I have seen the first one as well as the second. So, is there any difference?
Most monosyllabic (1-syllable) adjectives seem to use -er/-est to form superlatives.
Most trisyllabic (3 syllables) or longer adjectives seem to use the "more/most" construction.
According to various grammar sites, there's no clear rule for bisyllabic adjectives, with some suggesting checking the dictionary to see if -er and -est forms are listed immediately after the root word (but I don't think that's great advice because one can publish a more consolidated dictionary by just leaving those forms out).
In this specific case, "most tragic" is clearly correct and "tragicest" isn't, but I have a hard time explaining any rule why, beyond deference to a dictionary. The best I can offer by way of a generalizable test is this Ngram which shows "most tragic" in reasonably common use and "tragicest" not found in the millions of books Google indexed. That strategy can be used to figure out which superlative form other adjectives typically take.
As others pointed out already, most tragic is the correct form.
One-syllable adjectives use the -er and -est endings, and three-or-more-syllable adjectives do not, but there is no good rule that describes two-syllable adjectives. Some two-syllable adjectives use the -er and -est endings, such as happy/happier/happiest and feeble/feebler/feeblest, but others do not, for example famous and boring.
Note that for many adjectives, it is not really well-defined whether or not the -er/-est forms are acceptable. Sometimes native speakers will disagree. For example, once I translated a story into English and used the word tenderer. Tenderer is in many dictionaries and is attested in literature, and some of my editors accepted it without hesitation, but others said that it sounded awkward and insisted that I revise it to more tender.
When in doubt, you can always use the more and most forms. People will definitely notice if you say boringer or famousest, and they might even think tenderer sounds out of place, but I doubt anybody will react if you say more happy, even though happier is more common.
I also wanted to point out that if tragic had an -est superlative form, it would be spelled tragickest, not tragicest. The c in tragicest would sound like an s, and to my ear it sounds like a portmanteau of tragic and incest, or perhaps the superlative form of tragice (which is not a word).
Consonants are often doubled when a suffix is added (e.g. big/bigger, bat/batting, pad/padded). Whether or not a consonant is doubled will often change the pronunciation and meaning (e.g. hater/hatter, riper/ripper).
The doubled form of c is ck. This can be seen in the difference between slicing and slicking, and in words like magic/magicked/magicking.