Do you want to be close to the action? If so, Nana Plaza or Soi Cowboy? Nana is Sukhumvit Soi 4 and Cowboy is Sukhumvit Soi 23. Having stayed at Majestic Suites and Dawin Hotel on Soi 4, I can recommend these as girl friendly and nice hotels. I would ask them about taking more than one girl though - from memory Majestic Suites were fine with this but did not take more than one girl at a time for Dawin. Some hotels operate a 1 guest only policy and pay extra for the rest so best to ask. Also, if you book the hotel room for 2 people then that is also a good way to go.
I would suggest that you go to AGODA WEBSITE and search for hotel availability. Narrow down by location and amenities. Check the reviews from solo travellers for a good indication of girl friendly hotels. Pick one you like and is within your budget. Ask the hotel to confirm though on girl friendly position or try searching the hotel here on this forum to see if any historic comments have been made.
Good luck with the hunt.