I fiddled around extensively with the options and here's what I found to be optimal. Now, everyone's ears are different and you might find the need to fine-tune the EQ to personal taste, but you can start with these guidelines:
I tried every separate mode and wasn't pleased with the result from the others. Auto sometimes made my music / movies sound honky, with a slight bump in the mids. Movie and Voice modes have a weird mid scoop, which makes audio sound a bit "hollow" to me. Music seemed to be the best out of the bunch — and for every type of media, too.
Now on to the EQ. This one is going to be very personal for anyone, but here's a general tip you want to follow for Samsung phones — do not use the EQ to boost frequencies. Instead, cut down the undesired ones. Why? Because Samsung's EQ works kind of weird. When you boost a certain frequency above +0, it will start bringing all others down. This not only makes it a nightmare to actually tune in your desired sound, it also degrades the overall sound quality.
Without further ado, here's how I have mine set up. The middle frequencies are slightly scooped to get rid of that honkyness that small smartphone speakers are prone to procude. The very top frequencies are rolled off to get rid of some hiss.