Ho risolto, trovato la soluzione anche con un array monodimensione di seguito posto il codice per altri utenti:

<script type="text/javascript">

$(document).ready(function() {

// AutoComplete Server Data, Server Filter (List)
$("#txtAutocompleteServerList").autocomplete

({

source: function(request, response) {

$.ajax({

type: "POST",

url: "method.aspx/GetCountryList",

dataType: "json",

data: "{ 'filterKey': '" + request.term + "' }",

contentType: "application/json; charset=utf-8",

dataFilter: function(data) { return data; },

success: function(data) {

response($.map(data.d, function(item) {

return { value: item}

}))

},

error: function(XMLHttpRequest, textStatus, errorThrown) {

var errorMessage = "Ajax error: " + this.url + " : " + textStatus + " : " + errorThrown + " : " + XMLHttpRequest.statusText + " : " + XMLHttpRequest.status;

if (XMLHttpRequest.status != "0" || errorThrown != "abort") {

alert(errorMessage);

}

}

});

},

delay: 100

});

});

</script>

[System.Web.Services.WebMethod]
public static List<string> GetCountryList(string filterKey)
{

List<string> lstCountry = new List<string>();
lstCountry.Add("Albania");
lstCountry.Add("Algeria");
lstCountry.Add("Andorra");
lstCountry.Add("Angola");
lstCountry.Add("Anguilla");
lstCountry.Add("Antarctica");
lstCountry.Add("Argentina");
lstCountry.Add("Armenia");
lstCountry.Add("Aruba");
lstCountry.Add("Belgium");
lstCountry.Add("Belize");
lstCountry.Add("Bermuda");
lstCountry.Add("Bolivia");
lstCountry.Add("Botswana");
lstCountry.Add("Brazil");
lstCountry.Add("Burundi");
lstCountry.Add("Burkina");

var filteredCountries = from c in lstCountry
where c.ToLower().StartsWith(filterKey.ToLower())
select c;

//
return filteredCountries.ToList();
}