Naturally my answer will be about paraphrases, but in passing I think his etudes deserve some kind of mention. The one I've played the most often: Moses, especially as a concert-finisher where I only play the second half, ie the Dal tuo stellato soglio variations.
Other favourites: La Sonnambula, which I think is his masterpiece, Don Pasquale, La Traviata, which is imo one of the hardest and its difficulty is a matter of first hand experience... The L'art du chant is a lovely compilation mostly without obstructive difficulties (check out the Paul Wee recording which is imo quite a bit better than the other complete sets).
The early Robert le Diable is ferociously difficult (Ponti makes quite few cuts). It and La Traviata are my picks for the hardest out of those which have been recorded. The likes of Semiranide, Don Juan, Norma, Don Pasquale are of course also very challenging. I was recently made aware of the paraphrase on Zampa and it looks worth investigating.