I like still having my own old mtb, but it's just sentimental value- it's a terrible bike, as are pretty much all old bikes. I wouldn't mind a xizang, always had a thing for those.
But, what I would like back is my old BMX... We got it for £30 off a classified ad, and all I remember of it at the time was that it was indestructible, pretty light, and very high geared- I secretly never really liked it even though it'd have broken my dad's heart if I'd admitted it. But looking at pics now, it was a Tange Hot Heart, ex-Scotia race bike. Scotia BMX being where Chris Hoy started out. We just assumed all BMXs were kids bikes I think Sold it for what we paid for it but I'd pay a lot more now.