The most important oracle database performance metrics
Your question is too broad and opinion based for this forum. There are too many permutations and combinations. Start with the Oracle docco. Don't forget OS metrics - sysstat for Linux or see here for ideas about Windows (you didn't even mention your OS). 

Are you running an OLTP or an OLAP system? Different answers! Do you have a on-premise or cloud system? Different answers! What version of Oracle are you running? Different answers! How much are you prepared to pay (normally the crunch question!)? Different answers! 

There are any number of both free and pay monitoring software systems/frameworks - Nagios (free) and Foglight (Quest) would probably be the best known. 

Take the tour and check out this blog post - "Help us to help you". This site is a great resource and you'll get all the more out of it if you follow the guidelines. Give us a specific real-life scenario and then you should be able to get more assistance here. You could also check software recommendations.

[EDIT]

Maybe I missed the original reference to the Oracle Enterprise Manager (OEM) document. What you're really looking for is information on the Oracle Wait Interface (OWI), a complex but truly impressive piece of coding - not a system to be learnt over a weekend! You'll also want to delve into the mysteries of the Oracle Automated Work Repository (AWR) and the Automatic Database Diagnostic Monitor (ADDM) (may costo mucho bucks!).

The first thing that I would do if I were you is to get Christopher Lawson's "The Art and Science of Oracle Performance Tuning". It is a very (very! surprisingly so) readable introduction to Oracle performance tuning - he doesn't give you a "top 10" - he tells you how to look for one on your system. Then read, read and read some more (preferably till your eyes bleed! :-) ). Oracle is a huge system and, just as Rome wasn't built in a day, neither will a deep understanding of Oracle. But, if you persevere, you'll get here.