It's harder to answer than it sounds. There are many ways to translate "I say" into Latin. Some of the expressions convey subtly different meanings from others. Here you will find some of the main verbs used to mean "say" in Latin.
(I say, assert, affirm)
(I speak, talk, say, tell, mention, utter [loquor, lŏqui, -quūtus/-cūtus]
Used "in the tone of conversation" (Lewis and Short)
(I say, speak, utter, tell, mention, relate, affirm, declare, state, assert) [dīco, -ere, -xi, -ctum]
Lewis and Short refers to the following as synonyms: loquor, verba facio, dicto, dictito, oro, inquam, aio, fabulor, concionor, pronuntio, praedico, recito, declamo, affirmo, assevero, contendo
(I speak, say) [for, fātus]
Lewis and Short refer to the following as synonyms: loquor, dico, perhibeo, inquam, aio
(I say, assert, call, name [perhibeō, -ḗre -ui, -ĭtum]
This is a secondary meaning of perhibeo, according to Lewis and Short; its primary meaning is: hold out, extend, present, etc.