man, hard to choose just one, and my shop isn't even set up yet. My first thought was the aceteylene set, it was the first high-dollar investment I made. it is a Smith, and needs new O rings, but hasn't given me a moment of trouble. Except the dam bottles keep running out.. dam... Second tool to come to mind was the little 120V flux-core MIG. It splatters something awful, but is SO handy! the next tool to come to mind would definitely be my 4" angle grinder. With a cut off wheel, it surgically removes cancer, cuts exhast tubing to exact specifications, etc. With a grinder disc it smooths welds, eats bondo, rust, etc. With a knotted wire wheel it strips paint and bondo, and leaves an almost polished finish. But I think that my next major purchase is going to trump them all: an air compressor! it will be so nice to actually be able to use my many air tools!