1) In absolute terms (how much destruction the force could wreak, coupled with capability of platforms, etc.) 1988-ish - before the Cold War cuts, before the withdrawal of WE177, and when the Tornado was still cutting edge and credible.
2) Relatively speaking, you'd draw a different conclusion.
1955-ish.
Prior to the massive down-sizing of RAF Germany, disbandment of the 21 Squadrons of the RAuxAF, and the reductions in home-based fighters. Bomber Command was looking to get the V-bombers, and had a vast force of Canberras, which were still more than capable of evading interception, and delivering a 'hammer blow', albeit with US supplied nukes.
I'm puzzled for the votes for the early 70s - surely nothing had improved since the mid-60s, and there had been significant reductions in force structure.