Re: free database for java

So free databases for multi-platform, and java based are listed partly 
bellow :
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postgresql
mySql
Derby
h2 database.
open-office base.

I suppose the above are the most popular free-dbs,
and I think that mySql is the most popular of the five.

I found Derby, and I am learning using it.
I have installed Squirrel-sql, and I think it's good enough.

I can learn each of the other free-DB,
but it may be endless ...

Which do you think is the best of them ?

Thanks :o)