I have used a disassembler on the SNES (asmdev), and finding freeware for that
system was relatively easy, but getting info for the Sega Genesis 16-bit
68000 disassembler was time consuming and frustrating. Keep in mind that an
emulator that doubles as a debugger does not also mean that it can disassemble
binary. And a decompilar converts from binary to high-level code. So disassembler
is machine-to-low, decompiler machine-to-high. This guide is up-to-date
as of February 2022.

Systems that use 68k assembly:
Sega Genesis (game console)
Neo Geo (game console)
Commodore Amiga (computer)
Atari ST (computer)
and others

If you start looking at tutorials on the Internet about 68k you are bound
to come across Z80 assembly. The Sega Genesis uses a Motorola 68000 processor
for most of the computing which is where the Assembly language is named from. 
It has a Z80 processor that deals with audio. So if you are working on audio
from Sega Genesis roms you will need to be familiar with Z80, but 68k for the
rest. I.e. the op codes will be different from Z80 and 68k. It may be interesting
to note that the Sega Master System, the 8-bit console directly before the Genesis,
and Sega Game Gear, portable device, both use a Z80 processor for its main
operation (not just audio). 

List of popular disassemblers:
68kd
Ida Pro
Ghidra

I'm just going to focus on 3 in this short guide.

68kd:
Review from sonicretro.org-
"68kd is a command-line program by Bart Trzynadlowski which can disassemble
Motorola 68000 code. The output produced by 68kd cannot be recompiled by
SNASM68K [an assembler], and the program is unable to differentiate between code
and data, so its usefulness is limited. IDA Pro is considered to be a better 
alternative."

Link: https://segaretro.org/images/4/49/68kd.7z
(There is also a github with source code online)

Ida Pro:
Review from sonicretro.org-
"The Interactive Disassember, more commonly known as IDA, is a popular disassembler
used in reverse engineering software. It works on code generated for a wide variety
of processors, including the Motorola 68000 and Z80 processors used in the Mega Drive.
Although IDA performs a large degree automatic reverse-engineering to a certain 
extent, leveraging cross-references between code sections, knowledge of parameters 
of API calls, and other information, it is focused on being interactive. A typical 
IDA user will begin with an automatically generated disassembly listing and then 
rename, annotate, or otherwise add information to the listing, until it becomes 
clear what it does, creating an effective reverse-engineering.
Although at one time IDA was distributed freely by its creator (Ilfak Guilfanov), 
it was later turned into a commercial product by DataRescue, a Belgian company, 
who currently maintains and supports an improved version they call IDA Pro. A 
license for IDA Pro costs about 450 USD, but it is widely pirated, and at least 
two freeware versions exist."

Link: https://hex-rays.com/ida-pro/

Ghidra:
From their website-
"A software reverse engineering (SRE) suite of tools developed by NSA's
Research Directorate in support of the Cybersecurity mission."

Link: https://ghidra-sre.org/
(also open-source and on github)

More reverse-engineering utilities you may want to explore:
Hopper
x64dbg
Hiew
ODA
Binary Ninja
radare2