Examples with negative connotations:
* His thoughtless actions hurt her feelings deeply. (emphasizing the negative impact)
* She made a thoughtless comment about his weight, which made him uncomfortable. (highlighting the carelessness of the remark)
* The thoughtless driver left his car blocking the driveway. (focuses on the inconvenience caused)
Examples with neutral or less negative connotations:
* He was so caught up in his work that he was completely thoughtless of the time. (emphasizes distraction or focus on another matter)
* She made a thoughtless purchase, impulsively buying a dress she didn't really need. (describes a lack of planning or consideration)
* The company's thoughtless design disregarded the needs of disabled users. (emphasizes the lack of consideration for a specific group)
The best sentence for you will depend on the specific context and the nuance you want to convey.