a mco is a chess book on openings, it full name is Modern chess openings.
POLL: Do you like to use MCO'S or online Data bases?

I use MCO to find out what my opponents will play, then try to find busts to MCO using databases and more specialized opening books.

When I am out and about I like to pull my small chess set and my copy of MCO out of my pack to study. Harder to do with a database (and no wifi) so my vote is for MCO

When I am out and about I like to pull my small chess set and my copy of MCO out of my pack to study. Harder to do with a database (and no wifi) so my vote is for MCO
You can access databases online, along with a small chess set that you don't have to worry about losing the pieces. So you have access to databases on your cell phone.
Advantage: modern technology.

"Best" is relative. For ease of use, including how easy it is to update, Chessbase has a virtual monopoly on the database market. But there are plenty of excellent databases available to peruse online for free, including correspondence databases.
It's still used by a few old timers (it is a portable database after all)...
"Hey, they just mentioned me on the telly!"