<select name="imodel" id="imodelphone" onchange="func1(this)">
<option value="5">iPhone 5</option>
<option value="6">iPhone 6</option>
</select>
<script>
function func1(elem){
alert(elem.value);
}
</script>
Или
<select name="imodel" id="imodelphone">
<option value="5">iPhone 5</option>
<option value="6">iPhone 6</option>
</select>
<script>
imodelphone.onchange=function (){
alert(this.value);
}
</script>
Лучше так
<select name="imodel" id="imodelphone">
<option value="" disabled selected>Выберите модель</option>
<option value="5">iPhone 5</option>
<option value="6">iPhone 6</option>
</select>
<script>
imodelphone.onchange=function (){
alert(this.value);
}
</script>