1. Мыслить is right but it is used very rare.
2. РазмыШлять -great
3. Задумываться has a litlle bit another meaning. It is like to dive into smbd thoughts, to start thinking.
4.Обдумывать
5. Осмысливать
Colloquial : обмозговывать, ломать себе голову, раскидывать умом.
There are many synonyms but some are not interchangeable, so it is better to see a context in order to choose a proper verb.