City were knocked out 6-3 on aggregate, with Kylian Mbappe’s hat-trick sealing a 3-1 second-leg victory for Los Blancos.
Carlo Ancelotti’s side are the reigning champions, having secured their 15th European title last season, and Guardiola sees them as the frontrunners once again.
"Of course Madrid are a contender, definitely," Guardiola said after City's exit.
"But there are other good teams around. It will be an interesting Champions League season. Madrid are always favourites for this trophy. There are other teams who are having a very good season but Madrid are always favourites.
"But there are other teams that are playing very well. Liverpool are having a really good season up until now, Barcelona are amazing, Paris Saint-Germain, with my friend Luis (Enrique) in charge, are on the march.
“But obviously, Madrid are favourites."