[SalesForce] How to solve this error:-Unexpected character (‘<' (code 60)): expected a valid value (number, String, array, object, 'true', 'false' or 'null')

You are going to laugh at this, but it turns out that REENCODING as JSON at each step gets you your quoted values. Strange but true!! Try this:

String jsonData = '{"data": {"1500":{"id":"1500","last_name":"Test1"},"298":{"id":"298","last_name":"Test2"}},"status_code":200}';

//get top level map
Map<String,Object> jsonMap = (Map<String, Object>)JSON.deserializeUntyped(jsonData);

//extract data as JSON
String jsonSubset = JSON.serialize(jsonMap.get('data'));

//re-encode as a Map
Map<String,Object> jsonSubsetMap = (Map<String, Object>)JSON.deserializeUntyped(jsonSubset);

MyContact[] contacts = new MyContact[]{};
for (Object obj : jsonSubsetMap.values()){
    //re-encode as json, again!
    String jsonObj = JSON.serialize(obj);
    //deserialize that b@#$ard!!
    MyContact contact = (EBTestDeleteMe.MyContact)JSON.deserialize(jsonObj, MyContact.class);
    contacts.add(contact);
}

System.debug(contacts);

This outputs:

DEBUG|(MyContact:[id=298, last_name=Test2], MyContact:[id=1500, last_name=Test1])

This assumes you have a class called MyContact that looks like this:

public class MyContact {
    public String id;
    public String last_name;
}

Well, this is how you can do it. Apex is made through Java!

String jsonText = '{"Data": {"attributes": {"type": "Contact","url":"/services/data/v35.0/sobjects/Contact/0036100000JUXKkAAP"},"Id": "0036100000JUXKkAAP","Description": "","LastName": "Testing"}}';

Map<String, Object> cObjMap = (Map<String, Object>) JSON.deserializeUntyped(jsonText);
String cObjJson = JSON.serialize(cObjMap.get('Data'));
// Why again :(
Map<String, Object> cObjMapFurious = (Map<String, Object>) JSON.deserializeUntyped(cObjJson);
String cObjJsonDrunk = JSON.serialize(cObjMapFurious);
try
{
    SObject customObject = (SObject)JSON.deserialize(cObjJsonDrunk, Sobject.class);
    System.debug(' Accomplished: '+customObject);
}
catch(Exception ex)
{
    System.debug(' @@@@@ Don\'t visible '+ex.getMessage());
}