How to set the default Account Owner Value in Visualforce using Apex Class?

If you only want this defaulting to happen when the Visualforce page is used you can use a controller extension:

Apex Class:
public with sharing class MyExtension {
private ApexPages.StandardController sc;
public MyExtension(ApexPages.StandardController sc) {
this.sc = sc;
}
public PageReference save() {
Account a = (Account) sc.getRecord();
a.OwnerId = [select Id from User where Name = ‘umadevi’].Id;
return sc.save();
}
}

Visualforce Page:
<apex:page standardcontroller=”Account” tabstyle=”Account” extensions=”MyExtension”>