Task manager for a small company?

Question

Project management

Good afternoon! We are looking for a task manager for a small company, here are our requirements:
1. The ability to delegate tasks to other users is extremely important, as well as the ability to refuse execution with a comment (do not accept, because ...), that is, redirect and give someone to work .
2. Not too complicated interface.
3. Integration with google calendar masthead.
4. Obsessive reminder system.
5. Ability to attach documents to the task.
6. Ability to send notifications when the status/text of the task changes.
Please tell me the products that meet all these requirements.

Answer 1

Trello+Plugins

Answer 2

redmine

Answer 3

trello.com

Answer 4

trello

Answer 5

Todoist or trello