A lot depends on what is your purpose of studying the code? In other words, what does interesting mean to you?
If you are doing it in order to see how much complexity can go into a CPU, you may be more interested in studying the source of OpenSPARC architectures. It will take a long time to dive in but you will get an appreciation for the overall big picture view of a complex microprocessor.
Then, if it is to study specific computer microarchitecture features, you will want to look at some straightforward RISC machines like the AEMB, a small and fast multi-threaded 32-bit RISC processor (shameless plug).
Then, if what you want is to learn good coding styles and conventions, the LEON2 design is a good place to learn good VHDL coding style.
However, if your purpose in studying a processor is to learn how to design one yourself, your best bet would be to start with one of the simple 8-bit machines (there are many AVR, 8051, PIC examples on the net).