伦敦是我最喜欢的一个城市.
that 引导定语从句修饰a city 在定语从句中做like的宾语,可以省略.
I like London best.
如果句子是：London is a city that I'd like most to visit./that I'd like to visit most.
句中的that 一样可以省略,不过做的是visit的宾语.
不过,句子最好是：London is the city that I like best.特指.