A favorite of mine is Ghidra. Before they realeased+open souced it a few years back, the only option for real software reverse engineering (as in, for large and non-trivial programs, where you need more than just fancy disassembly) was IDA Pro, which is absurdly expensive if you're not sponsored or willing to pirate. Now, some of us kinda take for granted the fact that there's an open source world-class RE tool. And honestly, I even prefer it to IDA Pro - that's how good it is.