For me it would be a Compaq Presario (forgot model #) running Windows 95.

I don't even remember what model it was, I got it when I was like 6. It was a mid tower if I recall correctly running Windows 98.
Nobody in my family new how to use it and It was given to us by a friend. I still remember spending hours on it playing Treasure Cove
Good times.

The oldest computer I've OWNED was a Wang (yes, that was the actual maker) that ran Windows 3.1. The oldest computer I've ever used was a tape-driven monstrosity from the 70s that controlled a plastic press at my last job.

This doesn't belong in Microsoft Operating Systems. Moved.

Wow, I think I've used my brain before, but I might be mistaken.

This made me laugh. +1 internets.

Oldest for me is a TRS-80 Model 100 laptop from 1983. I own two

I own a Tandy 1400LT, but it has no power cord or operating system so I can't actually USE it.

Macintosh (1984)! Second oldest is some Tandy running Windows 3.1.

BBC Micro. God those things were awesome!

Oldest that I own is only a Pentium II class laptop, so fairly new in relative terms.

a PDP-11/34 last week (still in production use)

My parents started using computer before my brain was formed so the oldest computer I have used was not my brain. The first computer touched by my hands was a Pentium 4 2.4 Ghz desktop when I was SIX. Yeah I was just 13 now.

Mine would be an Atari 800 (I own two of them)

My second oldest would be my IBM PS/2 computer or my PowerBook 160. (Yes, I own both of them)

My oldest one i ever USED is my uncles 1996 Pentium.

A Compaq Deskpro PC from 1998. I don't have any old stuff.

The oldest computer I've owned is a Compaq Armada 1125, but the oldest I can remember ever using would be an original Macintosh from 1984.

IBM XT manufactured 1986

My oldest ever owned was an Mattel Aquarius from 1983. oldest ever used was an Apple ][ from 1979

EDIT My Aquarius only beats the IBM 5150 Clone by 4 months as oldest own.

Well, the oldest one that I actually own, strictly speaking, is a 1979 model of the Atari 2600, later known as the Atari VCS (Video Computer System), originally introduced in 1978.

I also have an 8 inch floppy disk (manufactured by Digital), with a copyright date of 1975, and some computer books and manuals from the 1960s and 1970s.

I own a ZX Spectrum 16K with the 48K RAM upgrade. I have touched a ZX80 which is older I believe.

The oldest I used and my parents had was an Philips P3238 (80286). After that (or actually besides the Philips but just some weeks later) there was an IBM PS/2 model 30(?) (80286).
When the Philips died it was not long after that a Tulip VisionLine dt 386SX (386SX) was the replacement.
After the IBM died there was an Compaq Prolinea 4/50 and a few weeks later an Compaq Prolinea 4/100.
Only the Philips my parents actually paid for... the rest were business leftovers.
The last one worth to mention is an old 486 server that was fitted with an new mobo and Pentium processor. It was an big-tower model with loads of bays and had our first HHD with 2GB (it was a Quantum Bigfoot).

From the list above only the Compaq Prolinea 4/50 made it (still own it)... all the others are gone after they died...
Although I like the 486 and older more than Pentium I still miss the big-tower. "Luckily" there is no way in rebuilding it, on one hand because I don't know the specs anymore and on the other hand because it was partly homemade by someone. I say "luckily" because now I can keep the great memory's with this machine as great memories.
The Philips is the one I'm actively looking for because it was a great machine for it's time with an brilliant design (The monitor (monochrome (B/W)) and probably the keyboard do we still own)

The oldest PC I own now is an IBM PC XT 5160. At the moment this IBM is fitted with an auto EGA/CGA graphics card and connected to the TV in the living room for games like Digger. The only hardware I need for the IBM is a game pads. Got some Trust infrared game pads but they are worthless...

Probably the BBC Model B.

Apple IIe... Need to get one of those...

Ok, electronic computers. The oldest I've ever used is an Altiar 8800 (seriously, those things are awesome). The oldest "Computer" I own is a Atari VCS (AKA 2600). The oldest more common computer I've used is a Apple II (original), or a TRS-80 Model 1.

The oldest real "computer" (not gaming unit) I own are actually 2 AT&T PC-6300s with 1984 date codes in one and 1985 in the other.

I have an Atari 2600 too!

Oldest I currently own/have used is an Apple ][+, but I'll soon be selling it, which will make my Osborne I the next oldest. But I'll probably sell that too in the near future, so the oldest I'll own is an IBM 5150.