The best area to stay in Sonauli is generally close to the India-Nepal border crossing, as this is the primary point of interest for most travelers.
This area is functional and serves as a transit hub, with hotels and guesthouses clustered along the main road leading to the border post. You'll find basic amenities here, including currency exchange services and small eateries geared towards travelers. Its main characteristic is its direct proximity to the border, making it convenient for those crossing into Nepal or arriving from Nepal.
For solo travelers or those on a quick transit, staying near the border crossing is the most practical option.