Ancient: HH St. Athanasius the Apostolic, HH St. Cyril I, HH St. Abraham (Abraam Ibn Zaraa, 10th century; one of the ethnically Syrian Popes of the Coptic Orthodox Church). HH St. Abraham was Pope when the miracle of the moving of Mt. Mokattam by St. Simon the Tanner happened, following three days of fasting together. We have kept those three days ever since that time, in remembrance of this miracle.
Modern: HH St. Cyril IV (r. 1854-1861), the so-called "Father of Reform" due to the huge impact he had on education in the Church, greatly raising the standards and bringing the first printing press ever into Egypt, which was received with the ceremony usually observed for the reception of relics. Unfortunately it was also during this time that the Coptic pronunciation reform began, under Arian Moftah (teacher of Coptic language at the Patriarchal church at the time), which as a linguist I can't really be too thrilled about for various reasons, but it was with at least a good aim in mind (proposed reunion with the Greeks in Egypt, which obviously never happened, apparently due to the meddling of the Ottoman viceroy). Obviously HH St. Cyril VI (r. 1959-1971), who is recent enough that I know people who met him. They all say that his epithet "Man of Prayer" was well-deserved, and there are many stories from his life which are told and retold that emphasize his humility and the indwelling of God in him. I believe them.
HH Pope Shenouda III and of course the current pope I think it is too early to say anything on. I will of course always have a bit of a soft spot of HH Pope Shenouda III, since I was received into the Church under him, but I am also not blind to some of his missteps in his writings (e.g., the whole dust up with Abouna Matta el Miskeen over Theosis, which showed that HH either didn't understand it or properly, or couldn't articulate his objections to it properly; I've read a lot on this, and I think a case could be made for either) and some of the problems the rapid expansion of the Church during his papacy have caused (e.g., the abundance of 'general bishops', a phenomenon which does not predate his predecessor, and hence can hardly be called 'traditional'), though how much of that should be laid at his own feet is above my pay grade to say. I pray that we return to our roots as a conciliar Church, as was the promise of HH Pope Tawadros II in one of his first speeches after his elevation. (Read: A lot of the problems 'created by HH Pope Shenouda III' have more to do with his extremely long tenure and how much the Church grew and changed during this time, and various political struggles faced in Egypt throughout his long reign, all of which somewhat changed the people's way of looking at the Pope into something that was closer to the modern RCC model than our traditional, less personally Papally-focused ecclesiology; Samuel Tadros writes about this at some length in his book Motherland Lost: The Egyptian and Coptic Quest for Modernity, if anyone is interested. It is indeed a modern phenomenon, and one that I know many people are working to turn us away from.)
From among the Latins/Romans, I have to say of course HH St. Hippolytus, whose writings I find myself consulting fairly regularly, and also HH St. Felix, and St. Clement (all of these are commemorated in our syanaxarium, along with several others).