The Isar Card 9 Uhr, formerly the Grune Karte, a month's travel on Munich's public transport network
If your schedule allows to you use it then this ticket is the one for you. It is valid for one month, for Euro61 you can travel anywhere on the S-Bahn, U-Bahn, trams or busses as often as you like with one simple restriction. You may not use the ticket between 4 and 9 on weekday mornings. Of course, that's not so great if you need to be at work at 08:30 but many companies are happy about staff who will turn up as soon after nine as their commute allows.
There are some more bonuses too. Public holidays count as weekends and have no restrictions in the mornings. You can take up to three children under fourteen years with you and, if they are your own children and under fourteen, you can take as many as you like with you. Also, on weekends and public holidays and after six in the evening on weekdays, you can take along one adult as long as they have a valid season ticket for some specific journey on the network. The idea being that, in a traditional working family, dad has a monthly season ticket from home to work and mum has an Isar 9. That allows dad to travel to work, mum to take the kids with her during the day and the whole family to travel freely in the evenings and at weekends.
You can get a version of the Isar 9 Uhr Card that is valid for just the inner part of the network or just the outer part but the saving is not great and, if you need to make, for example, one journey a month to the airport and back for a trip to the UK, you may as well get a card for the whole network.
You can buy these tickets from the MVV ticket desks at the main station or from the D-Bahn ticket desks there as well as at the Ostbahnhof and at the airport. Also, the automated ticket machines at the Hauptbahnhof and some of those at the Ostbahnhof and other stations can dispense these tickets. You just press the button for an MVV Monatskarte and follow the instructions from there. It's no cheaper than queueing up at the ticket desks but it can be faster. One tip would be to buy your ticket for (say) March on about the 20th of Feb. That way, you do not have to join the end of the month queues.
Because it is valid for a specified month, this ticket does not need to be validated. Also, they are completely transferrable so you may lend your ticket to a friend. One last bonus is that a card is valid up until noon on the first working day of the next month. So, if the last day of the month is a Friday, you can still use the card over the weekend and on Monday morning.