Asm One seems to be a memory eater, it takes about 500 KB for itself.
My favourite assembler is still PhxAss written by Frank Wille (phx). Don't know why he didn't suggest his own excellent and fast program here? I use it together with PhxLnk and PhxAssGUI. It's great!
PhxAss update to v4.46:
http://eab.abime.net/showthread.php?p=994297#post994297
PhxAss full version 4.39:
http://aminet.net/package/dev/asm/PhxAss439
PhxLnk v4.32:
http://aminet.net/package/dev/asm/PhxLnk432
PhxAssGUI v2.1:
http://aminet.net/package/dev/gui/phxgui
The GUI needs some small fixes for WinUAE (it tries to read from address -4).