Shiva Excursions is the best trekking company in Nepal. I have gone on multiple trips with them including Langtang, Chitwan, and Annapurna Circuit. The guides are much more professional than at other companies, and you can expect clear hassle-free communication. I have always felt safe and cared for on my trips. Additionally the guides are knowledgeable and can help you build skills for more advanced mountaineering. I always travel with Sudip Adhikari as my guide, and we have become great friends. The experience you get trekking in Nepal is life-changing. You can see glaciers, ancient villages and gompa (monastery), talk with locals and learn about Nepalese mountain culture.
My personal favorite trek in Langtang, as it is quite close to Kathmandu and offers stunning views of the mountains and glaciers. On the last trek to Langtang, Sudip took us up to Langtang Lirung basecamp. We were able to observe hundreds of waterfalls pouring off the glacier, and yaks in the alpine meadow.