I think we need to know the year first. I mean are you looking for period correct so that you can blend in?
This magic watch that was, apparently, never manufactured anywhere or purchased by anybody, but was still owned by two different people, nonetheless:
Go back to 1957. Buy ten chronographs that Omega just started selling. Omega calls it the “Speedmaster”. Yeah, I know, the name is kind of hokey. But just buy them. Trust me.
When this baby hits 88mph you're gonna see some serious shit. Don't forget your extra shot of plutonium. You'd hate to get stuck whenever you're going.
You know I made a comment about this when that James Franco mini-series came out (11/22/63) - he's going back in time and buying beef and then selling it in the present for a healthy profit. Which just strikes me as beyond stupid when there are so many things that are worth far more. And ounce for ounce I don't think the value is any higher than with vintage watches.
Make sure to not wear the ca-50 like Marty did, the electronic beeps can really give you away if you go back to as far as let's say, 1955. Alternatively if you want to prove you are from the future this might be a good choice!
watches? naahh I would be looking at specific companies and get several hundred of their shares. Then, all my desires for wrist watches will correct itself.