Best database choice for a highly scalable web ticketing system?
Scalability is factor for every business that expects large growth. Ultimately only you and your team can answer the question with whatever is right for your pocketbook and time considerations. The best we can do is offer some options.

You could use a master MySQL database for the writes and then implement replication to create multiple, read-only slaves.

Or MySQL for the write database and then create a custom ETL application to convert the transactional data into read-only and place it into a NoSQL database like Cassandra. I've found Cassandra to be optimal for tons and tons of reads (which is great if people are performing searches on your content), but it will require you to be pretty savvy in the database realm.