The 35-year-old played with a whole host of huge names across his stellar Wales career but he is convinced no side comes close to the class of 2012 when it comes to mental strength.
"I think mentally it's the toughest bunch of guys I've been involved with and I think their age is a big sign of that," Williams told BBC Radio 5 Live.
"They're inexperienced and there's a no-fear attitude there.
"They just want to go out and play rugby and aren't really that fearful of losing games, which is great, it's refreshing."
"These youngsters were going to the World Cup to win it and I've never experienced that before in a Welsh side.
"Mentally they are very tough and they really, really do believe they are world-beaters, which is great."
Wales take on a Barbarians squad that will feature Williams at the Millennium Stadium on Saturday June 2 (kick off 2pm) before touring Australia later that month.
As well as taking on Super 15 side the Brumbies, Rob Howley's men face Tri Nations Champions the Wallabies in a three-Test series in Brisbane, Melbourne and Sydney.
Williams sees the summer adventure as the perfect breeding ground for future success, although he admits it will be a tough test of that mental resolve he previously highlighted if the first Test goes the way of the hosts.
"You've got to learn to play the bigger, better teams in the world and experience losses," added Williams, who signed off from his Wales career with a last-gasp try against Australia the last time the two teams met in Cardiff.
"What's going to be interesting on this tour is that I don't think Wales will go there and win 3-0. It's going to be a massive ask.
"It'll be interesting to see how they go there, perhaps if they do lose their first game, how will they bounce back from that and go on from there."