Over at the Parchment & Pen blog, they’ve started a new interesting series called “Top Ten Theologians” (throughout all of church history). I added my personal top ten but divided them into two lists:
1/ from antiquity to the Reformation; and
2/ Modern era
(and NOT in any particular order because that would be too difficult). Here they are:

From antiquity to Reformation era:
Augustine
Aquinas
Luther
Calvin
Irenaeus
Basil
Ambrose
Origen
Tertullian
Anselm

Modern era:
Karl Barth
Norman Geisler
J.I. Packer
R.C. Sproul
Jonathan Edwards
John T. Mueller
Helmut Thiellicke
Wolfhart Pannenberg
C.F.W. Walther
[added:]  N.T. Wright (forgot the tenth)