Interesting post, HC.
While I can agree on the importance of the E major Trio by Mozart, I equally love the C major K.548 too. A magnificent work with the most telling slow movement from all six of the Wunderkind.
However, Haydn's Piano Trios are not vastly important because of the cello line (which, however, is musically well placed always), but owing to the brilliant Piano writing, the perfect form and some of the most memorable melodies and harmonies. I particularly appreciate the A major, the F sharp minor, the last E flat major, the E major and the serious d minor, among the incredible wealth of his Trios. However, even the early ones are so brilliant in writing and musical ideas, like the one in c minor.
For the most eloquent role of the cello in the Piano Trios, I'm afraid one has to go to Beethoven at least (see the glorious op. 70 in D major, with this magnificent d minor middle movement (the "Ghost"), and, by all means, to the Piano Trios by Schubert, arguably the best in the genre.
Surprisingly, the Piano Trios by Hummel are some jewels to discover, with roots both on Mozart and (less) on Beethoven.