I like to look up a lot of classical chess matches, and every then and now I run across some that are quite instructive. I can't say that I have a favorite one, because my reasons for looking at old games varies. Sometimes I'm looking for aggressive opening play (Morphy, Diemer, Tal), while other times I'm looking for solid positional/defensive play (Steinitz, Petrosian, Capablanca). Perhaps to my detriment, I prefer studying old games as opposed to studying chess books. Give me a collection of games (especially with commentary from the players) anyday!
Have you run across any old games that you found to be instructive? If so, then post a link to that match here with a brief description of what you learned. Can't post a link? Don't know where to find old games? No problem. Just post the game itself using the 'insert position or game' icon being sure to give credit where credit is due.
I'm looking forward to this running forum. Hopefully we can get as much participation in this one as we get in the others.