If the assembler that comes with Storm C isn't good enough then try Barfly. Barfly consists of an assembler and a debugger, both of which are probably the best you can run directly on Amiga.
SAS/C has a good IDE with both an editor and debugger, covers C89 and some basic C++, and its assembler and linker are perfectly fine.
There's also vbcc, a portable C compiler for C89 and C99, which comes with a really powerful assembler and linker.
Stay away from all-in-one-total-assembly-experience programs like Devpac, SEKA, Maxon Assembler, ASM-One with derivatives etc. They are really, really great for testing and learning, but for actually writing programs they only let you dig a hole in the ground for yourself.