My first recollection of trying emulators was FusionPC 3.0 for DOS running Mac OS 8.0 just like a real Macintosh! I remember being astounded that you could change the desktop resolution on the fly from within this emulator when Windows 3.1 could only do this with a powerful (for the time) graphics card and the very latest drivers installed. (FusionPC was doing it using a generic VESA 2.0 driver for DOS.)
At around this time I discovered DOSUAE 7.5d and had this running Amiga OS 3.1 just like a real Amiga, too. It was quite a while before I discovered that the reason I could only emulate df0: and df1: floppies was because I didn't have enough file handles declared in my DOS startup configuration! Constant ADF-swapping made OS 3.1 so much more long-winded to install.
Later, I managed to install Amiga OS 3.9 in WinUAE 0.8.8r8 running under Windows 95 using a procedure written by Thomas Rapp and available from his home page. In fact, Thomas continued to recommend this version of WinUAE in preference to later versions in which to install Amiga OS 3.9 for quite some time, and I found it worked very well.
By the time later operating systems came along, I had been well and truly bitten by the emulation bug, but it was really my experiences with emulators under MS-DOS version 5.0, when I had no other machine but a Pentium 133 PC, which converted me into the multi-platformer I am today.
So, my vote has gone to 'Other'.