Hardware wise? Amiga 500 Bat Pack. I loved that computer, it opened up a whole new world for me.
PC hardware wise? My original 3DFX Voodoo PCI - I got this on a whim at a computer fair (prior to the explosion of the internet and online retailers like OCUK, we had micro mart and trade fairs), I had no real 'need', but it quite literally transformed my gaming experience in Quake/Quake II. A close second goes to the Microsoft Optical Trackball, I purchased this about 16? years ago and it was so good I got another not long after, it allowed me to game again for extended periods (wrist problem) and work. I now use the wireless logitech equivalents. Honourable mentions also go to my dual c300a set-up, I was rocking a Supermicro P6DBE with them overclocked to 504 at stock voltage with a pair of MSI sockets and FOP32's with a minor mod when everyone else was buying Pentium 2's for silly money, Crucial M4 for obvious reasons. I suppose my MacBook should also get a mention, I suffered inferior alternatives to for years as I begrudged the price tag, I couldn't have been more wrong. It has and continues to be a perfect tool, apple has always done right by me, 5y/o screen replaced FOC earlier this month same day as I raised the issue without any agro.
Software wise? Win2K to leverage SMP properly as before that it was NT3.51/4 and multithreading was not a thing (you relied on the OS's task allocation to the least utilised PU) or Win7, an OS that has been superb for so long. Corel Draw literally made me hundreds of thousands of pounds over the years, I am grateful.
) to power it for gaming. As a result will always keep I think my primary monitor in that sort of aspect ratio and had it until I picked up one of the first ASUS PG348Q's which is largely the same but 100hz + G-Sync.