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Your current code generates an INNER JOIN, which as you've noticed, excludes any Applicant without an ApplicantAddress.

You want to select from Applicant and generate a LEFT JOIN on ApplicantAddress, this SO answer shows how to use DefaultIfEmpty() to do that:

var query = from u in usergroups
 join p in UsergroupPrices on u equals p.UsergroupID into gj
 from x in gj.DefaultIfEmpty()
 select new { 
 UsergroupID = u.UsergroupID,
 UsergroupName = u.UsergroupName,
 Price = (x == null ? String.Empty : x.Price) 
 };

It's hard to tell exactly what/how to change in your code to make it work, because we're not seeing the DbContext and it's not clear how Request turns out hitting the database; I'd rather not say anything than assume what's going on.

In this specific case:

private static Expression<Func<Request, RequestResultModel>> requestResultExpression = request =>
 request.Applicant
 .ApplicantAddresses
 .DefaultIfEmpty()
 .OrderBy(address => address.IsPreferred)
 .Select(address => new RequestResultModel
 {
 Id = request.Id,
 ApplicantName = request.Applicant.FullName,
 ReviewerName = request.Reviewer.FullName,
 RefferrerName = request.Refferrer.Name,
 City = address.Address.City,
 Province = address.Address.Province.Code,
 DisciplineCode = request.Discipline.Code,
 Event = request.Event,
 StatusName = request.Status.Name,
 Submitted = request.Submitted
 })
 .FirstOrDefault();