You should practice everything, but the single most important thing is reading broadly in my opinion.
The reason is because well written texts contain more intelligent forms of the language.
All spoken spoken language is a "dumbed down" version of it - written language will teach you much better.
If you practice only the spoken language, you will never write well, and never speak particularly intelligently.
So, aim high and keep reading beautifully written texts, as it will lift your language sense to higher levels.