We are blessed to have a Pope who is also a great theologian. More and more of his works are being translated into English; if you don’t read Latin or German I’d start with these.
Historically, you can’t go wrong with Augustine or Aquinas. “City of God” by the former and “Summa Theologica” by the latter are must reads; I also admire Augustine’s “Confessions” and sermons for insight into the Christian life.
I have Thomas a Kempis on my Kindle reading list at the moment; he is quite highly regarded as well.
Among modern theologians, John Cardinal Newman is uniformly excellent. Chesterton and C.S. Lewis are always interesting, and if you take your apologetics straight Hilaire Belloc is bracing
Among contemporary theologians, Anglican N.T. Wright on the Resurrection is hard to beat. Father Richard John Neuhaus is terrific and very entertaining (see his ecumenical journal “First Things”). I heard Father Benedict Groeschel speak at the Eucharistic Congress in Charlotte last year and found him to have very profound insights without a trace of pedantry. (I would be very surprised to see the word “pedantry” appear in his writings, for example).
And yes, Sheed is very widely respected.