I thought about doing a general thread about Great Generals at one point, but I think it's highly situational. A couple of examples: if you either have no religion, can't adopt a religion, or just don't have theology early on, how to get twice promoted units? Easiest is to put a GG as Military Instructor in a Heroic Epic city - provided of course you do have Heroic Epic built. Late in the game a Military Instructor might help you get your next promotion, but if you already have enough experience to go near that promotion and you have military tech advantage maybe it's better to build a Military Academy and pump those troops faster. They'll get promoted anyway after their first fights.
Some other things I learned after a few tries:
If you plan on domination with intercontinental wars it's good to have a coastal city with Military Academy if you already have Pentagon or can run a +2XP civic, or a Military Instructor in a coastal city, especially if the war is fought pre-docks. That lets you promote your transports with flanking and navigation to faster reach the other continent.
If you're in a big war (or plan on having a series of wars) multiple Medic III units are really valuable. At most I had two up to now, but sometimes I could have used even three. It's better to have another Medic III than giving a unit 13XP instead of 11XP with the next promotion at 17XP anyway.
And there are even situations where I consider dropping the 20XPs on multiple units. One time I was in a war with Huayna and needed a quick way to heal some troops that were going to be attacked next turn, so I shared the XPs between them and promoted those I could. It did indeed help. Another game with the Romans saw me share the 20XPs between Praetorians to get a few of them to the next level. I didn't finish that game unfortunately, but I think at the moment it was better than settling the GG. And in my last game I'm playing the Chinese and I attached the GG to several Cho-ko-nu's. One more drill promotion for them is pretty cool.