What are the best defenders now?

Depends on meta.

If in an area with high turnover where you need to maximize your minutes: 1. Blissey, 2. Snorlax, 3. Nothing else even matters.

If in an area with super low turnover: As weak as possible.

If somewhere in between: Who knows?

Communication.
As you don't need number of gym to get rewards, but period of time, one gym for each should be good. Talk with other team, distribute some gyms, all will be happy.
According to current system, no matter what you put into, I can take it down in 15 minutes. But why should I do it, I already have one which give me enough coins everyday.

As long as it is not one of your 100 best attackers... anything works as long as nobody will want to feed it berries. Rats work. Or Sentret.

The best defenders are ones that;
- aren't above 3,000 cp, because they rapidly lose motivation
- aren't pushovers, because they will get knocked out too soon
- aren't rare, because your teammates will keep feeding them berries
- aren't too intimidating, because you want the other teams to kick yours out after some time

With that said, Vaporeon seems like a very good fit imo. Arcanine and Houndoor also look good. Definitely won't put my Tyranitar, Dragonite, Snorlax, Blissey, or Machamp in a gym.