Because sometimes you want to go where everybody knows your name
On a quiet side street off of Boulevard Montparnasse @ 17 Rue Brea
The wine. All their wines are organic and delicious. The owner will help you pick a wine based on your preferences. Wines are only sold by the bottle so come thirsty. They do have small plates of cheese and meat if you need a nibble.
For a less lively vibe, come during lunch hour. You can even camp out there and get some work done on your laptop...only creative stuff like writing or drawing because even I find it questionable trying to do any other kind of work while drinking. During the evening, any hour is good, lively and fun. It does get packed by 8pm and is super tiny so if you are dying to go, you may want to get there on the earlier side.
I recommend going with at least one friend since you are purchasing a bottle and it's honestly more fun that way since this is a true neighborhood spot. If you do come solo, you can make friends easily by sharing your bottle. If the wine is just TOO good and you choose to be greedy, you can take the rest home. Mostly locals come here so I recommend being able to speak basic French. The man who works there does speak English, but he makes you work for it :)