The best lounge experience with any Delta partner is, by far, the Virgin Atlantic Clubhouse at LHR. But LHR isnt an A350 suites destination. And you have to pay the UK APD surcharge to fly in business out of the UK. (Plus, Virgin-operated business class is terrible - worst seats on any Delta TATL partner, now that KLM has finished refitting their 330s.)

Actually, the lounges at most of the suites destinations are all pretty bad. AMS and ICN are both barely worth a visit. NRT’s sky club is fine, but not spectacular. PVG’s MU lounge is credibly passable but I wouldn’t make a special trip for it.