Arizona:Phoenix
Arkansas:Little
Rock California:Sacramento

List<string> lstResult = new List<string>();
        using (StreamReader sr = new StreamReader(@"C:Stackfile.txt"))
        {
            //string test = sr.ReadLine();
            string line = string.Empty;
            while ((line = sr.ReadLine()) != null)
            {
                lstResult.Add(line.Split(':').Select(x=>x).Skip(1).SingleOrDefault().ToString());
            }
        }

        comboBox1.DataSource = lstResult;