Hmm, the scenario is that somehow you can only have one weapon?
Well, then I guess that means that it would have to be a small (2.5-3.5" blade) folder since I always have one in my pocket. It would also mean that my pepper spray must have fallen out of my pocket, and that I'm not at home and can't get there or I'd have 4 hunting/camping knives to choose from, two multi-tools to choose from (one larger, one pocket sized), a machete, a big Maglite and soon a small camp or backpacking axe.
In a more realistic situation, other than firarms I'd go with one of my defensive folders (doesn't matter that much which one) in my pocket and one of my hunting/camping knives on my belt. I'd leave my Maglite (too big to carry around easily, though a smaller one might end up in the pack). The Gerber camp or backpacking axe I'm getting in the next few days would be either in the pack or even on my belt (depending upon how low profile I wanted to be). I'd probably have the bigger multi-tool on my belt and I might even have the smaller pocket one as well.
Now in keeping with the spirit of your question, any bladed weapon, but only one, as your main/only weapon...I'm not sure. The knife is easier to hide (keep a low profile) and a hunting knife could be pretty effective. However, I'm thinking that the small backpacking axe could be pretty darn effective, and it wouldn't be that hard to conceal (it is fairly small).