It's really a matter of personal choice. But let's face it, a
pond filled with ogons is going to be somewhat boring after a while?
I like a good collection of go sanke (including doitsu go-sanke, shiro's, gin rin and tanchos) with a good yamabuki ogon and/or asagi or shusui to offset things. A good metallic like a kujaku is also a very nice'to have. A nice big ochiba or Chagoi is also a must as they relax everybody once they get calm and big.
so in my world you need a BIG
pond!