Which is the best exit from metro Montparnasse to get to Avenue du Maine northbound direction?

The RATP Android and iPhone apps have station surroundings maps where exit numbers are shown. There are also neighborhood maps in the station itself (for a station with multiple exits, there's usually one on each platform and one near a cluster of exits, but not one at every place where you need to choose between multiple exit corridors).

For some reason, the station surroundings maps on the website no longer show metro exits (they used to a few years ago). The website maps do show the exact location of bus stops, and connections to buses are clearly indicated in the station — but in your case I think the best exit doesn't lead to a bus stop.

A street map shows that your hotel is northwards of the station on Avenue du Main. To get to the north part of Avenue du Maine, take exit 2.

Note that this exit is on the metro 6 & 13 side of the station, the one at the end of the railroad tracks. It's a bit of a walk from the other end of the station where lines 4 and 12 stop.