Learn English – What does ‘a lunch best forgotten’ mean

There is the following sentence in Jeffery Archer’s fiction, “The Fourth Estate.”:

He droppped into three newsagents on the long walk into Kingston, and
purchased Time, Newsweek, and local papers. He then stopped at the
first restaurant he came across, with American Express sign on its door, took a
quiet table in the corner and settled down for a lengthy lunch.

Having paid for a lunch best forgotten, Townsend left restaurant and
began to stroll around the town. It was the first time he had spent
like a tourist since his visit to Berlin back in his student days.” –
P436.

What does ‘a lunch best forgotten’ mean? Does it mean the lunch he ate was terrible rather than mediocre, and worthless to remember, or he almost forgot to pay?

According to Google Ngram, the incidences of usage of ‘best forgotten’ is significantly lower than ‘best remembered’ by almost 10 digits (best forgotten: 0.0000046 – 48 vs. best remembered: 0.000032 – 28 during 1998 through 2008), and I don’t think I’ve met this word so often.

Is ‘something best forgotten’ an idiom to mean the thing you’d better to forget, like ‘a lost love best forgotten’, ‘a failure best forgotten,’ and ‘one’s ex-wife best forgotten?

I learnt there is a hit song, “Better Best Forgotten” by Steps released in March 1999 with Wikipedia, but it doesn’t seem to be relevant to “a lunch best forgotten” here.