[Dilettante_Pro]

<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>