Only one??
Ok. Now that we've got that behind us, I would purchase an all-arounder. For most people, that would be a blade between 6" and a max of 8.5".
If we're talking Scrap Yard, then I guess that would be a S6. You might still find a reasonably priced one. Maybe a Yard Guard or a Yard Hook. You should be able to find any of these for less than $150.
Swamp Rat? Maybe an M6 or Mini-mojo ($150-$200) but
I would more STRONGLY recommend a Chopweiler. You can still find them at the Ratknives for $198.95 retail.
Budget going higher? Look at a CG-ASH1 from Busse. With the heft of the fat CGs, you've got a pretty good all-arounder again. Other options include a Fusion Steel Heart (FSH or HHFSH,) a Hell Razor or SFNO, maybe a SJTAC. The range for these other options are $400 - $700.
Just my two cents. It's worth less than that, actually. For my *only* blade, it probably wouldn't be a dedicated EDC or heavy chopper.
If you decide on a different blade, as long as you don't
grossly overpay, you should be able to resell/trade it and get a different "one blade." Something like an ergo NO or SH, maybe a SHSH, the skies the limit! Just like your credit card limit should be! (Yeah, right. Only one... LOL)