MUC has no observation deck, but they have 2 visitor hills. At one hill, there are a Super Constellation (Lufthansa) a Ju52 and a DC3.
If you want to take pictures this place is not very good.
There are several spots around the airport, but major construction work is done at the moment, so the airport is undergoing changes especially regarding photopositions.
Here is an example of a picture taken at MUC:
Click for large version
Photo © Mathias Henig
This picture was taken at the taxiway between the southern runway (08R/26L) and the apron, but this position is closed now. But there is an equal position at 08L/26R.
Check my homepage for a detailed description: http://www.spotter.de.vu
And keep in mind: a ladder will help you at MUC