Can I have some classical music suggestions?

mylo asked the question:

I continue to hear about research done proving that listening to classical music while studying can actually make you focus better, and that it is the only music to not hinder your learning abilities while studying.
I’d like to give this a shot!

Can someone point me in the right direction?
I would like some good classical music to start off with. Maybe something nice and pleasant? Nothing too dramatic, startling, or distracting, but anything that is classical music will be fine!

So please, start me off in the right direction with classical music!