[SalesForce] Getting an option as default value in a lightning-combobox component

Couldn't you evaluate this just before creating the component? Just check for your object name, and then instantiate. Like so:

var thisCriteria = component.get('v.currentCriteria');
var comboboxValue = thisCriteria == null ? 'selectedObject' + index : thisCriteria.objectName

$A.createComponent(
        'lightning:combobox',
        {
            'aura:id': 'objectSelect',
            'id': index,
            'placeholder': 'Select Object',
            'label': label,
            'value': comboboxValue,
            'options': options,
            'onchange': component.getReference('c.handleObjectSelect')
        },

Though I am late in answering this question, but here is the other approach to populate combo box values.

Controller

Here retrieve UserRole records based on SOQL query and no data transformation in this class

public with sharing class UserRoleController {

    @AuraEnabled (cacheable=true)
    public static List<UserRole> getUserRoles(){
        return [SELECT Id, Name FROM UserRole];
    }
}

js Controller class

In this class, inside wiredUserRoles() method, perform data transformation for value and label.

Use, roleOptions() getter property to return the array.

/* eslint-disable no-console */
import { LightningElement , wire, track} from 'lwc';

//import method which should be used from Controller
import getUserRoles from '@salesforce/apex/UserRoleController.getUserRoles';

let i=0;
export default class DisplayUserRole extends LightningElement {

    @track items = []; //this will hold key, value pair
    @track value = ''; //initialize combo box value

    @track chosenValue = '';

    @wire(getUserRoles)
    wiredUserRoles({ error, data }) {
        if (data) {

            //create array with elements which has been retrieved controller
            //here value will be Id and label of combobox will be Name
            for(i=0; i<data.length; i++)  {
                this.items = [...this.items ,{value: data[i].Id , label: data[i].Name} ];                                   
            }                
            this.error = undefined;
        } else if (error) {
            this.error = error;
            this.contacts = undefined;
        }
    }

    //gettter to return items which is mapped with options attribute
    get roleOptions() {
        return this.items;
    }

    handleChange(event) {
        // Get the string of the "value" attribute on the selected option
        const selectedOption = event.detail.value;
        console.log('selected value=' + selectedOption);
        this.chosenValue = selectedOption;
    }

    //this value will be shown as selected value of combobox item
    get selectedValue(){
        return this.chosenValue;
    }
}

HTML

<template>
    <lightning-card title="Display User Role" icon-name="custom:custom63">
        <lightning-combobox
            name="Role"
            label="Role Name"
            placeholder="Choose Role"
            value={value}
            onchange={handleChange}
            options={roleOptions}>
        </lightning-combobox>
        <p></p>
        Selected Value:
        <lightning-formatted-text title="Chosen Key" value={selectedValue}></lightning-formatted-text>
    </lightning-card>

</template>

Output

After selection, display the value.