Personally there are two figures that help me to think theologically. And thinking theologically to me has to be constructive, as you already know (!).
That means such thinking should not merely drive us back to church history but to open up discourses where theologies are being construed afresh and relevantly explored.
These two figures are N.T Wright and Rowan Williams. Nowadays, I suspect that sooner Mike Higton, Oliver O'Donovan and John Webster will be the third, fourth, and fifth ones. (Don't know why Anglican theologians are so attractive).
Though Alister McGrath's Scientific Theology is exciting, nuanced, and very helpful in theological thinking but his works lack immediate relevance to society as compared to Wright and Williams.
I tried Robert Jenson, but he seems to be somewhere on the clouds. As for Norman Geisler, you can forget about him. I got 3 out of his 4 volumes of Systematic Theology texts, and thank God I didn't get the complete set. I would opt for Grudem and Erickson rather than Geisler. I've J. Calvin's Institutes and Cornelius Van Til's Intro to Systematic Theology but still couldn't find time for them. On the other hand, I appreciate John Frame's perspectivalism which he developed through his Theology of Lordship series.
But when it comes to Systematic Theology textbooks, I wouldn't recommend works by the two greats, simply because they might appear to be too 'constructive', hence controversial and easily misunderstood. And I don't think 'controversial' is healthy for those who are interested and just starting to pursue understanding.
And I benefit not only through books but through various people who thread on this same passion.