None.

Honestly, no Star Alliance J lounge in Europe is so noteworthy as to try and route your flight through it. Take whatever flight is available at the times you want to go.

If you must have an answer, however, Turkish @ Istanbul is probably the best, but presume that’s out of the way for what you’re looking for. UA’s lounge at LHR is not too bad (can’t believe I am writing that), but connecting at LHR T2 is a pain (long walks) and the lounge is not quick to get to/from if departing intra-Europe. LX is ok in ZRH and the non-Schengen GVA wing, but still quite basic.

if you’re flying F, then LH and LX do have great lounge experiences.