There's not such a thing as a 'must own' in my opinion, what's a 'must own' for one person is a 'would never own' for another. Watch collecting depends so much on your personal style it's hard to call what someone else should buy, so my choice below is entirely based on which affordable watch I plan on buying next as it fills a gap in my collection.
So...
If you want a mechanical watch which is value for money, is interesting to look at and isn't the same as what everyone else has for a good price, you might have a look at a Seagull 1963? You may think it's hideous or you may love it, but when you consider what you get for around $200 (I think, I'm in the UK so prices may vary) I think it's a great piece to start a collection with.