Silver, No Team Gym
Okay, you've just taken down an enemy gym -- if you hope to get some coins, which mon do you leave in it? Do you leave one that can be prestiged but isn't a pushover? Or the biggest, baddest one in your storage?
You don't have any teammates with whom you plunder -- just random people on the same team, who may or may not notice this new gym. (So no bubblestrat.) Ideally, you'd like your mon to last a few days. Say, too, that revives are growing kind of scarce.
As for me, I might leave one of my B-Team defenders:
2000CP Exeggutor with Confusion / Psychic
1900CP Tauros with Zen Headbutt / Horn Attack
2000CP Vaporeon with Water Gun / Water Pulse
1900 Venusaur with Razor Leaf / Petal Blizzard
1600 Clefable with Zen Headbutt / Moonblast
1900CP Gyrados with Bite / Twister
Also, do you prestige it yourself before you leave?
Answers
With teams of 6 now being able to take down gyms, I find it unnecessary to leave a low CP poke. its easier to run through a gym with a 2500 gyarados vs 3-4 vapes than one with a 500cp nothing squashed in there (basically no gym xp). Just leave a your best B guys in the event that another teammate may come along and help you hold down a gym for an extra day or two. if you know you are going to lose it or if its your last gym and you know you will definitely lose it within the next day, just leave a pidgey or weedle.