What is the most classic keyboard shortcut for switching between input sources (languages)? What is the most classic/used keyboard shortcut for switching between input sources (languages)?
Except for cmd+space of course which is used by Spotlight.
If you want your shortcut don't interfere with spotlight you can use option + cmd + space or something else.

but as you said cmd + space is really good as myself use this and replace control + space for spotlight, and use option + space for QuickSilver.