I've been using DevPac since the early ninties, it's clean and not cluttered, configurable, compiles very fast and is in my opinion the best M68k assembler ever on the Amiga.
Devpac is a great assembler package that set THE standard. There are some other assemblers that have surpassed it in some ways. Many of them are also free where Devpac is not. Examples of FREE Amiga assemblers...
vasm
http://sun.hasenbraten.de/vasm/ + actively supported, retargetable, and free
+ best optimizing 68k Amiga assembler
+ Devpac and PhxAss compatibility as well as many other assemblers
+ very powerful and flexible
- bigger and slower than assemblers that offer less
PhxAss
http://aminet.net/dev/asm/PhxAss.lha + easy to use
+ good optimizer
+ fast
- some bugs and quirks in uncommon 68020 instructions
Asm-One
http://aminet.net/dev/asm/ASM-One.lha + fast
+ popular
+ all in one editor and debugger is fast and powerful
- all in one editor and debugger is old and dated
- a few bugs
Barfly
http://aminet.net/dev/asm/BarflyDisk2_00.lha + very fast
+ excellent optimizer with control of each optimization
+ debugger, BDebug, in the package is very powerful
- not the easiest to use
- not popular
