Ultimate = (1) being or happening at the end of a process ; final ;last.

• Their ultimate aim was to conquer the topmost position in the business.
• The boss will make the ultimate decision who to employ.
• .
• (2) the best available of its kind.
• This restaurant is described as ” the ultimate in luxury”.
• He is believed to be the ultimate painter of this century.

Penultimate = second from the last ; last but one in a series of things.

• The penultimate chapter of the book.
• It’s the penultimate episode of the serial.

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Antepenultimate = third from the last.

• The antepenultimate item on the agenda.
• X is the antepenultimate letter of the English alphabet.