air-france-bus (one bus-option ends right at pjaucutt's Grand-hotel/InterConti at the Opera) leaves all terminals every 10-20 min
train/metro, depending on the length of your stay in downtown Paris, and on your transportation needs there, buy a NN-day-/week/month-tickets for the zones you will need for your travels (valid also for bus). THIS IS ALWAYS MY CHOICE (if I have a luggage-"problem" - then I might take a taxi after arriving by train in downtown Paris.
Don't buy the touristic-1/2/3/4/5/7-day ticket, buy the NORMAL weekly one (for that one you need a "carte orange" which is free and for instant use: have a passfoto for this). The prices are very reasonable (and a return-bus-ticket from the airport with air-.france costs about the same as a weekly-ticket on train/metro/local bus!).
taxi.