Your perfect debugging emulator.

[tankendog]

The biggest thing Id like is step by step instruction execution, and a memory dump. It was fairly easy to dump registers, but a memory dump would have been amazing when I coudln't figure out why my DMA I wrote was not accounting for the offset I calculated. It got really frustrating to the point of one of my professors offered to help write a debugger with me to be usable for 65816 code. 

[FAST6191]

As soon as any emulator pretends to even have something it can call debugging features it will usually at least have run to line which can be used in functionally the same way/to allow stepping. Likewise even without debugging features most of the cheat making tools (emuhaste, artmoney...) will be able to dump the memory for you.
Granted the better ones will also display register states at that point which can make things a bit easier.

That said fair enough, a discussion on the perfect debugging emulator probably also ought to include what is the bare minimum necessary to be useful.

[cret]

your stuff should be stable, and not segfault when an external debugger attaches to it