Quick answer: The S-Bahn (lines S1 and S8) is the fastest reliable option — about 41–45 minutes to Munich Hauptbahnhof, with a train roughly every 10 minutes combined. The Lufthansa Express Bus and taxis are the alternatives.
S-Bahn (train)
S8 takes about 41 minutes to Hauptbahnhof, S1 about 45 minutes — since they run on alternating schedules, you get a departure roughly every 10 minutes combined. This is the most predictable option regardless of traffic.
Lufthansa Express Bus
Runs from both terminals every 20 minutes, 06:25–22:25 daily, taking about 45 minutes to the city center. A one-way ticket costs €12 online (€13 if bought from the driver).
Taxi
A taxi takes about 35 minutes but is significantly pricier and less predictable — published fare estimates from different sources range roughly €50–€110 depending on traffic, time of day and which company you use, so it's worth asking for an estimate before you get in.