Day to day, simple variations make a difference: “Thanks a lot”, “Thanks so much” and “Cheers” (very British) sound natural and warm among colleagues and friends.
In a professional setting, raise the tone: “I really appreciate it”, “I’m grateful for your help” or “Thank you for taking the time” convey respect and care.
And to reply to a thank-you, skip the repetitive “you’re welcome”: try “My pleasure”, “Happy to help” or “Anytime”.
In practice
Casual“Thanks a million!” is a friendly exaggeration among friends.
Professional“I truly appreciate your support on this.”
Reply“My pleasure. Let me know if you need anything else.”