The most common database used is MySQL, because it's free and well-documented.
However, if this database is truly going to be *HUGE*, then I'd suggest you spend a lot of time doing research before you choose a database.
A lot depends on what your definition of huge is. A hundred thousand records may be huge to you, but to most databases, that's not very big.
You've got a *lot* of choices in databases. One of the biggest factors will be whether your product is something that would allow you (or the purchaser of your product) to buy the database in question.
Some of your choices are:
Free:
Unix DBM files
MySQL
PostgreSQL
Commercial:
FileMaker
Frontbase
Openbase
Valentina
Sybase
Oracle
Wade