1. Best One-Sentence Summary: I am convinced that the Catholic Church conforms much more closely to all of the biblical data, offers the only coherent view of the history of Christianity (i.e., Christian, apostolic Tradition), and possesses the most profound and sublime Christian morality, spirituality, social ethic, and philosophy.There are more, but these are the few I saw and liked the most.
2. Alternate: I am a Catholic because I sincerely believe, by virtue of much cumulative evidence, that Catholicism is true, and that the Catholic Church is the visible Church divinely-established by our Lord Jesus, against which the gates of hell cannot and will not prevail (Mt 16:18), thereby possessing an authority to which I feel bound in Christian duty to submit.
3. 2nd Alternate: I left Protestantism because it was seriously deficient in its interpretation of the Bible (e.g., "faith alone" and its missing many other "Catholic" doctrines - see evidences below), inconsistently selective in its espousal of various doctrines of Catholic Tradition (e.g., the canon of the Bible), inadequate in its ecclesiology, lacking a sensible view of Christian history (e.g., "Scripture alone"; ignorance or inconsistent understanding of of development of doctrine), compromised morally (e.g., contraception, divorce), and unbiblically schismatic and (in effect, or logical reduction, if not always in actual belief) relativistic.
Disclaimer: I don't therefore believe that Protestantism is all bad (not by a long shot - indeed, I think it is a pretty good thing overall), but these are some of the major deficiencies I eventually saw as fatal to the "theory" of Protestantism, over against Catholicism. All Catholics must regard baptized, Nicene, Chalcedonian Protestants as Christians.
8. Catholicism avoids theological relativism, by means of dogmatic certainty and the centrality of the papacy.
10. Catholicism formally (although, sadly, not always in practice) prevents the theological "pick and choose" state of affairs, which leads to the uncertainties and "every man for himself" confusion within the Protestant system among laypeople.
14. Catholicism retains (to the fullest extent) the elements of mystery, supernatural, and the sacred in Christianity, thus opposing itself to secularization, where the sphere of the religious in life becomes greatly limited.
In Our Lord and Our Lady,