I have used MINIX, NetBSD, Linux (Debian), and Amiga's UNIX AMIX on different models. The fastest and bestest(haha) is Minix. Check it ot at
The Amiga MINIX Page. It is only text based, but it is good enough to get the feeling of a UNIX system.
[EDIT] I will try to locate the MINIX 1.5 disks on the Internet and include the link in this post. MINIX for the Amiga will run on the following.
* The harddisk driver works with the A590, A2091 and A3000.
* MINIX can use all A3000 32-bit fastmem.
* MINIX runs on 68000, 68010, 68020 and 68030 processors.
[EDIT] MINIX 2.0 did not come out for the Amiga. For the complete MINIX 1.5 Disk Images, Documentation, and updates to 1.5.10.2 use this link
Aminet's Search for MINIX