Williams, who was left out of the first Test by Lions boss Sir Clive Woodward before being called upon for the other two matches, said he was glad to be home as the Lions returned to Britain.
"It's good to be back: it's been a tough tour and it has taken its toll," the Wales flyer admitted.
"But I enjoyed it, it was the experience of a lifetime and certainly the toughest tour I've been involved in with the toughest games of my career. You can only learn from that - it was a great experience which I will cherish forever."
Williams rejected any criticism of the tour, insisting the players' commitment could not be faulted.
"We went out there with the right attitude, the right way to play rugby with a great squad, and at the end of the day it just did not come off on the field. We're very disappointed but no-one can take the blame for it," he told Sky Sports News after arriving at Heathrow Airport.
"I said before coming out here they (New Zealand) are the best team in the world, especially on their day when they perform like that. They have been almost untouchable. They have some great players and they take a lot to beat."
Wales centre Gavin Henson, who was also not selected for the first Test, said: "It was a long tour and it was difficult, it wasn't the best weather and we didn't get the results we would have liked but it was a really good experience.
"It was great to be involved with some world-class players, I think I have learnt a lot and I can't wait to use all that next season."
Regarding the first-Test omission, Henson played down the disappointment he revealed while on the tour.
"He (Woodward) explained why he didn't pick me and I understood - that's the way it goes, it's a difficult game we are in," he said.
"I had my chance in the second Test and got injured after five minutes and don't remember much of it so it's disappointing the way the season ended. But it's been a great season."