"Seb had an unbelievable year and in the end dominated significantly," Planet F1 reported, Horner told BBC Radio 5 live.
"But there are some big opponents out there, with six World Champions on the grid.
"Lewis is going to be pushing hard this year, Jenson had a great year last year, you can't rule out Fernando because he is a world-class driver and it will be interesting to see how Kimi fares on his return."
When pressed for who - aside from Webber - he expected to provide the biggest challenge, Horner responded: "Probably Jenson."
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