Ich liebe dich
- serious, true love (husband to wife, boyfriend to girlfriend),
- also used in strongly connected families
Ich hab dich lieb
- Medium, not too definitive love or
- strong, but not yet too settled and meaningful connection,
- also amongst family members
Ich mag dich / Ich hab dich gern
- good friendship,
- amongst closer relatives,
- notice: can be used to neutrally positive opinion about objects (tools, food, clothing, ...)
Notice that all these mean actual love or strong attraction, not just good worker's fellowship. People of same genders using these terms on each others could appear homosexual (especially amongst males, but not so much if the statement clearly has no context to love because of difference in age, hierarchy borders, ...), although normally understood. In such situations, you would somehow need to figure out another way of telling them that you like their humor, appearance, style or much appreciate their work and so on. Generally, if there's no true or rising love, there's no need to declare your feelings that openly, unless you have to decide about important things that are influenced by how well you get along with each other or are confronted with a reproach that tells wrong about your feelings.