Nilsson. Please ;)
Nilsson's Isolde is fantastic. Some people say that she's not sensitive enough, but I think she's just the best - as Isolde, Bruennhilde, Turandot (and maybe many others, which I haven't listen). Recording with Nilsson from 1966 owns also truly wonderful Tristan - Wolfgang Wingassen (my favourite singer; he sings III. Act with such energy, sensitivity, sense of theatre and dramatic situation) and great Brangaene by Christa Ludwig (My God, I will hear her this Saturday as Narrator in Schoenberg's "Gurrelieder"!).
Very interesting topic. I've also written something about Verdi vs. Wagner, but it's in Polish and it isn't good enough to translate it ;) Verdi is great composer, sure, but I prefer Wagner: his stories are better (and with complex, symbolical characters), his life and personality are more fascinating and, well, his music is just more moving and beautiful for me; last but not least, his operas are in German (which is - besides Polish, of course - my favourite language) and he's often quoted by Thomas Mann, my favourite writer ;)
What a pity that we have very little of Wagner's music in Warsaw - I must write a petition or something like that.