Ireland captain Rory Best "is the best version of himself he has ever been", says ex-Irish centre Darren Cave.

Best, whose team begin their World Cup bid against Scotland on Sunday, has faced criticism in recent weeks over his line-out throwing and it was suggested his captaincy may also be under threat.

"I don't know what Rory has got to do to win people over," said Cave.

"For some reason he has to constantly keep proving himself."

He added: "I don't think there has ever been a rugby player that has had to prove themselves so many times - but he is such a belligerent guy, it probably brings out the best in him," added the 11-times capped international.

Best will retire from playing after the World Cup but Cave believes the Ulster hooker is still a huge influence on the squad, as well as meriting his berth in the starting line-up.

"Some people say he's past it but he is as fit as he ever was. His tackling, his leadership, the impact he has on other players and his work at the breakdown all make him a key component of Joe Schmidt's team.

"He is Ireland's most successful captain and people respect him for the way he conducts himself and how hard he trains, but also for what he has achieved."

First up for Ireland in Pool A on Sunday is an encounter with regular Six Nations opponents Scotland in Yokohama, followed by games against Japan, Russia and Samoa.

If Ireland progress from their group, New Zealand or South Africa are their likely quarter-final opponents as they attempt to progress beyond the last eight for the first time.

Cave said: "When the draw happened a couple of years ago, it looked like Ireland had a free run but since then Scotland have improved massively and Japan have had some fantastic results.

"We know the physicality the Russians and the Samoans have, so it has turned out to be quite a tricky group.

"The way Scotland play will suit Ireland I think. The way to beat Ireland is through absolute brute power, like England did twice this year and like Wales did in the Six Nations.

"Scotland will try to move the ball a lot, which will really suit Ireland. While Scotland are a good side, I think Ireland have the beating of them."

'All Blacks favourites'

Cave, who retired at the end of last season after making 229 appearances for Ulster, makes New Zealand favourites to win the tournament for a third consecutive time.

"I think the All Blacks are favourites. They are the team I'd be looking to avoid - outside of that, it's an absolute lottery.

"I wasn't massively impressed by South Africa or Australia but they made me sit up and take notice by both beating New Zealand comfortably in the Rugby Championship.

"England are very, very strong but Ireland showed the kind of form in their last two warm-up games to give us hope. I think we are on the right track again."