Here is an excerpt from Mahabharata for the proof that Vibhishana being given a boon of immortality.
Saying, 'It shall be so' 'Brahma then addressed Vibhishana, 'O my son, I am much pleased with thee! Ask any boon thou pleasest!' Thereupon, Vibhishana replied, 'Even in great danger, may I never swerve from the path of righteousness, and though ignorant, may I, O adorable Sire, be illumined with the light of divine knowledge!' And Brahma replied, 'O scourge of thy enemies, as thy soul inclines not to unrighteousness although born in the Rakshasa race, I grant thee immortality!'
Now my question is, why did no Rakshasa (like Malyavan & Sumali) consider Vibhishan to be the most worthy to rule them as he is the only descendant of the Rakshasas that gained immortality. He may have become an ally of Rama later on but in the beginning of his life early on he used to support Ravana and help the Rakshasas. Since he was given immortality he should have been more powerful than even Ravana.