Okay.. Not the oldest,just the one in your collection, that makes people question your sanity..

The one that gets questions like "What the hell is it" or "You paid MONEY for that?"

Mine is a 1905 Ross carbine. Fifty bucks, shoots great, and has a neat pattern of pits forming a spiral like rifling in the barrel..
All I can say is. it's a lot of fun..

Meplat.

I've got a flintlock pistol from 1730. Never fired it, but my grandfather remembered shooting it when he was a kid.

Edited to say that most of my gun collection is Civil War and WW2 rifles, so that pistol really stands out.

Early Bushmaster pistol,made in the 1970's.I paid $40 for it 2 years ago.

Well its not the oldest, but I have a "Buffalo Classic" by Harrington and Wesson, it is in 45-70, single shot that weighs 10 pounds, cresent steel butt plate, kicks like a mule and always draws attention at the range.  

Hmmm.  Howbout a Portuguese Mauser-Ver_____?  The odd mauser with the split rear bridge - the bolt only has a half dozen pieces, but unless you know how to put it together you cant...

The Steyr-Hahn M1912 pistol.  It looks very blocky looking with stripper clip loading, huge rear sight bridge, and rotating barrel.  BTW, this one was "rechambered" by the Germans in the '08 parabellum cartridge.  They ran a tapered reamer into it and throated the barrel with a vengeance.  Regular 9mm cases come out VERY pregnant looking.  I gave a couple fired cases to a CHP instructor if she did not say where she got them.  I use only 9mm magnum brass cut down to 9mm largo length in this one.  Very smooth integral magazine and never a failure to feed.

How about the Astra 400?  Very underrated gun.  It is fun to freak out people at the range by alternately loading in a magazine 9mm, 9mm largo military, 9mm largo JHP handloads, and regular 38 acp loads, and firing groups.

Did I mention the VZ 52 rifle in 7.62x 45?  It was once issued to someone named Arashi ---- (forgot).  Rusty rifle with a sewerpipe bore, but groups 5" at 200 yards.

I could go on for a long time.  Jeff could count on selling me any weird surplus gun that came through the door, and did so for 3 years.

How about a straight pull Ross model 1910 in 280 Ross?  Guys make it sound like I am living on borrowed time when I admit shooting it.

This is the Killer of the Ross line.  It has a rotating bolt head like a Lee, but on a straight pull.  It is easy to put the bolt into the receiver incorrectly, and close the bolt on a cartridge and fire it unlocked.  

You have to watch what you are doing, that's all.  I still will never lend out this rifle because of this (or for that matter, any other rifle again!).