Considering the whole opera; Sullivan's gifts and skill are evident not only in the sheer quantity of lovely melodies, and the unsurpassed deftness and inventiveness of orchestration and accompaniment; but in a seamless quality of cohesion to the score - which moves from one delight to another and also has a distinctive pastoral quality throughout.
Plus, Gilbert's wit and satire are as good as anywhere; and somehow retain their freshness despite massive changes in society since the height of the Victorian era. For an English audeince, there are several 'belly laughs'.
As a young man I was a good actor but only an adequate singer; and the part of Strephon in Iolanthe was the most significant role I ever did. There isn't a massive amount of singing to do as Strephon (he does not have his own aria, for example); but for much of the opera people are singing about him; because the plot hinges on the fact that he is half a fairy (literally, his upper half... down to the waist) - his mother being the immortal Iolanthe, and his father a mortal man whose identity is unknown until the end.
The excellence of Iolanthe reaches its peak in the Finale to Act one which is 20 minutes of continuous and extremely varied music; never less than very good and at times supremely good. The marvels fly-past thick-and-fast... enough to provide highlights for three or four normal, 'successful', musicals. The deftness and elegance of the musical passages and accompaniments is, throughout, at the highest level of skill and fluency.
Here is the version conducted by Malcolm Sargent from 1958 (which I have on vinyl). Among the many excellent performances, that of Monica Sinclair as the Queen of the Fairies stands-out as being among the very best bits of contralto singing I have ever heard anywhere from anybody!
If you want to follow the Libretto, a version can be found here: