What you've asked is a difficult question since it depends on what philosophical problems you find compelling. Since newer philosophers tend to respond to previous ones' ideas you might have difficulty just diving into, say, Hegel or Kant. Nietzsche is a bit easier to read since he tries to be accessible and interesting. And he's my favourite to read. A lot of his political ideas may be controversial, the one of the things that I find aligns with my own ideas is how he saw emotion and rationality as intertwined, and inseparable. That becomes a critical part of explaining how the mind is able to think inventively later in this series.

If you have any problems that you are interested in, I could probably point you in the right direction.