It is difficult to pick a favorite market in Paris - some have the best prices, others have higher quality or more unusual products and a few have simply incredible ambiance. But if I had to pick one market in Paris to be the "best" market for first-time visitors to the city, I would pick the Marché Richard Lenoir.
This renown market is impressive in both its size and the diversity of its products. Stretching north of Place de la Bastille (under the watch of the famous monument's gleaming angel), this twice weekly market fills a fountain-lined promenade with a motley crew of both vendors and shoppers. As you enter on the Bastille end, you will walk past cheap clothing and hygiene/beauty products, followed by kitchenware merchants. You will then see stands of prepared/hot foods, fruits and vegetables, and eventually dairy, meat and seafood. Once deeply entrenched in the market, specialty vendors of Italian goods, honey, spices or wine will also dot your path. (Word to the wise: it is worth walking the entire loop of the market before deciding on any purchases. And a line typically means that a vendor has good value and/or high quality products).
There are two elusive and addictive foodstuffs sold at this market that I have never found of equal quality elsewhere in the city. The first of these is fougasse, a doughy webbed bread, that I prefer stuffed with black olives. This particular Parisian delight is an obsession of my bread-loving sister (who, ironically, doesn't like olives, but apparently loves olives encased in perfectly fluffy, soft bread). The second time I lived in Paris, my apartment was steps from the Richard Lenoir market - and I can actually recall waking up at the crack of dawn, rolling my suitcase to the bread stand (before they were even officially set up), and purchasing still-warm fougasse, just to hail a taxi and hop on a plane back to the 'States - just so she can have it (relatively) fresh. (Yes, it's really that good).
The second of these products is less portable, unfortunately. Pain au thym is a lebanese flat bread spread with olive oil and za'atar - a middle eastern spice blend of thyme, marjoram, oregano, sesame and salt. Heated over a cast-iron dome, the circular flatbread is then folded into parchment paper, piping hot and ready to eat.
After thirty seconds of impatience (which are necessary, I have in overeager moments burned my tongue), the fragrant bread is ready to bite - inundating your taste buds with an herbaceous, salty and slightly acidic punch. An empty stomach is an undisputed prerequisite for such a market trip, but filling that stomach immediately with pain au thym more than gratifies the short-term sacrifice (and may help inspire moderation during the rest of your shopping experience).
Last but not least, this is a market well-worn by savvy tourists, and thus easier to navigate for English speakers than others (for example, the nearby Place d'Aligre market, which is very popular and often preferred for daily shopping by full-time residents of the quartier).
If you have the chance, check out the Richard Lenoir market early on a Thursday. It is far less packed than it will be on Sunday, and thus easier to grab the elusive fougasse (which tends to sell out in the first couple hours).