Hmm.
I actually really like C.S. Lewis, what little I've read of him.

Tillich is really good because he makes a point of placing himself and his ideas within the Christian tradition, but also examines what that means (he doesn't take for granted that certain symbols about God are going to be understood in the same way he does, and so he explains everything). His Systematic Theology is very very large, but also comprehensive and coherent. Courage to Be is apparently the book that made his reputation, and it's pretty small, so if you want to start with Tillich that would be a good point.

I like Schubert Ogden and Charles Hartshorne for a very conversational writing style that nevertheless contains arguments. They are not as clear in their thought process as Tillich, but they are interesting for being part of a movement called Process theology that views God as informed and changed by human actions, and that God works subtly to encourage humans to choose good actions.

I mean, CS Lewis is actually a good place to start. You might just try and look for references in his works to authors he likes, or find people who talk about his work- there's a bit of a field right there.