Do not take the RER Smirnie
I can't recommend the best route, but trust me, do not take the RER.
I thought it would be quicker than a taxi and I was running late from a meeting. The first challenge was to find the RER in Gare du Nord, then I boarded the most horrid train I have been on in years. All the seats looked like they had had vomit spewed over them at some point in their life, and whilst clean the stains were still visible to the eye.
The train travels through some vile suburbs of the type frequented by North African immigrants. And on the trip to CDG the train stopped twice in these god foresaken hovells; well you can imagine your fellow travellers. I'm sure these two ruffians had flick knives and would have gone for my Tumi and cashmere coat if the train wasn't packed to the gills. By some miracle I arrived safely at CDG. Not at the terminal but some bus depot on the outskirts of it. There was a shuttle bus waiting and it only took a few minutes to reach the check-in desks. No seat on teh bus of course; I was strap hanging.
Never did I deserve a G&T in the lounge more than that day. Sometimes I still wake up in the night in a sweat from the nightmares of that journey.