If someone says that you are the ultimate salesman, smile and say thank you. If they call you the penultimate salesman, don't take it as a compliment. They just said that you are second best. (Actually, being in the top two out of the millions of salespeople on Earth wouldn't be that bad an insult.)
It is not a very common word in English, but penultimate is one of the most commonly misused words. The reason for the confusion is that it looks similar to the related word ultimate. However the meaning of the two words is different.
Ultimate describes something as the 'most extreme possible'. Being the Chief Executive is the ultimate government job in Hong Kong. Ultimate can also be negative, someone who is the ultimate failure cannot do any worse; nobody else is as badly off as them. Penultimate describes something that is next to the ultimate. The letter Y is the penultimate letter of the alphabet.
There was once a British TV series (Danger Mouse) where the next-to-last episode was predicted to be the best episode ever to occupy that position in a series, so the announcer urged viewers to "Tune in next time for the ultimate penultimate".
A certain badly written, but famous, British history text incorrectly claimed that The Battle of Bosworth Field was the penultimate battle of the British civil war. It was the most important battle and it ended that war. It was not the second most important.
Too many people like to use the word penultimate, because it is a big word that sounds clever. However when they use the word incorrectly they will sound foolish. There's nothing wrong with using big words, like penultimate in your homework, but make sure that you use them correctly. If you're not completely sure check a dictionary or get someone to proofread your work.