It depends on if I'm trying to stick to Japanese cannon or mythological cannon.
Bahamut is pronounced "BaHAAmoot"
Quezacotl is pronounced "ke-tsih-ko-ah-tl"
Ragnarok is pronounced "ROON-ya-ruhk", but that sounds ridiculous so I usually just say "Rag-nar-rock"
Now many of the words are changed when the game is translated, (Cactuars are called Sabotenda in the Japanese versions), but the ones that aren't are east to figure out the pronunciation of by looking at the word.
Seifer is pronounced like the word "Cypher" (Germanic languages have "ei" pronounced like we pronounce the word "eye").
Chocobo is properly pronounced "Chokkobo".
Tidus is pronounced "Tee-doos", but that's very awkward for english so "TEE-dus" is probably an okay approximation.
Kjata I can't QUITE remember, but I believe the proper pronunciation was lost in translation (similar to how Typoon should have been spelled Typhoon), and that it's really "KOO-jah-tah".