LightningSelect.html

<lightning:layoutItem flexibility="auto" padding="around-small">
    <div class="custom-box">
        <lightning:select value="{!v.Opportunity.StageName}">       
            <option value="">Choose one...</option>
            <aura:iteration items="{!v.picklistValues}" var="item">
                <option value="{!item.value}">
                    {!item}
                </option>
            </aura:iteration>
        </lightning:select>
    </div>
</lightning:layoutItem>