Translation:You are the most famous person I have met.
The "mai" isn't required. It's more often used (as you can see I use the "mai" instinctively in my examples, but it's not obligatory).
You can find also some examples in the Treccani Encyclopedia:
"la più grande rivoluzione che la storia del libro abbia conosciuto"
And on my opinion there is also a difference in meaning between "che io conosca" and "che io abbia conosciuto"
Yes, you're right, it's not mandatory, but more often than not you will hear "che abbia mai conosciuto" instead of "che abbia conosciuto".
The difference between those two versions exists purely from a grammatical point of view. The meaning is acutally the same: "abbia conosciuto" indicates somehow a succession of people you have come to know during your lifetime, but that also means you still know them right now (since you can't un-know someone) which is exactly what the expression "che io conosca" eventually means: "...that I know (right now)".
It's great to learn these nuances - thanks to you both, brigister and sandrabruck. Though I can't help sensing a 'feeling' or 'poetic' difference in the two, even if the meaning is strictly speaking the same. "abbia conosciuto" sounds more emphatic, more heartfelt - perhaps because there is that sense in it of "ever in my whole lifetime" here?
Is the difference in meaning between "che io conosca" and "che io abbia conosciuto" something like the respective difference in English between "whom I have known" and "whom I have met"?
I ask this because I have noted that the perfect form of Italian conoscere, because it relates to a completed action, is generally best translated as "have met" rather than "have known", while on the other hand the English uses the perfect tense "I have known him 2 years" where Italian - like French and German - uses the present ("Lo conosco da 2 anni"). Shifting these into the Subjunctive mood would give the two forms in question above.
I too looked for mai, cf. French "Tu es la personne la plus célèbre que j'aie [subjunctive] jamais connue." Surely this is somewhat implausible with "tu"...By the way, French speakers have to be careful not to make conosciuto agree in gender with the object, as is required in French.
Yes, normally the participle doesn't change when the perfect tense is formed with avere, , but I thought there are some exceptions like in French: In French the participle has to be accorded when it follows the noun it refers to, as usually is the case with relative clauses: "The pizza I've made" -> La pizza que j'ai faite I thought it would be the same in Italian: La pizza che ho fatta Any Italian could clear this up?
The reason because "conosciuto" is masculine even if "persona" is femminine, is because the verb (in active form) doesn't change in gender or in number, exept when the direct object is a pronoun. In this case the direct object is explicitly expressed in the sentence: "ho conosciuto la persona" (who have I met? "la persona") you usually can't change the participle in number or gender whan the direct object isn't a propnoun. In the other sentence, "ecco le pastiglie. - le ho prese!" (what did I get? "le") the direct object of "prendere" is expressed with a pronoun and this is the reason because you decline the participle in gender and number.
- "siete i bambini più belli che io abbia conosciuto", "sei la donna più bella che io abbia conosciuto", etc. the participle doesn't change in number or gender;
- "lei è Maria, l'ho conosciuta molto tempo fa" (l' = la), "hai visto i fratelli Rossi? li ho conosciuti alla festa": the direct object is a pronoun and goes before the verb. this is the case in which we change the participle in gneder and number;
- "ciao Luisa, felice di averti conosciuta", "care sorelle, vi ho conosciute bene": again, the direct object is a pronoun, so the participle change in number and gender.
"abbia" and "sia" are the conjungation of the auxiliary verbs.
for active forms:
Transitive verbs (like "provare") always take "avere" as auxiliary.
Intransitive verbs about movement (like "venire") often take "essere" as auxiliary.
Other intransitive verbs (like "vivere") could take "essere" or "avere". And, sadly, there isn't a rule.
for passive and refrexive forms: the auxiliary is "essere" (intransitive verbs can't have passive or reflexive form)
for conjungation, in general: http://www.effectivelanguagelearning.com/free-language-lessons/italian/grammar/verbs
for the use of "essere" and "avere": http://www.learnitaliandaily.com/en/italian-grammar/italian-auxiliary-verbs-to-be-and-to-have
a list of verbs which take the auxiliary "essere": http://www.oneworlditaliano.com/english/italian-verbs/essere_as_auxiliary.htm
if you want to know if a verb is transitive, you should ask yourself if the question "what?"/"whom?" has any sense, but you have to do it with the italian structure: there are some verbs which are transitive in English and intransitive in Italian (for example "like", "miss", etc: "I liked you"=I subject, like transitive verb, you direct object; "mi sei piaciuto": tu implied subject, sei piaciuto intransitive verb, mi=a me indirect object).