What's wrong with John Oliver apart from being an LFC fan? Haven't watched it in a while now but the pieces have usually been well constructed, documented, and funny (the latter being subjective of course), and over the years he's built his own mythology with characters and running gags.
His topics have run out of steam over the years, imo (not his fault, but still).
He's also nowhere near as intelligent or as strident as Stewart (the interview on TPWJS in yesterday's episode was brilliant).
You're right that the relative quality of their humour is subjective. But I personally think Oliver is miles behind and relies on the same punchlines over and over. He's become a bit of meme in that respect these days.
Oliver blew up in popularity during the Trump era as one of the leaders of the leftwing media resistance. His prominence has dropped since Biden took over. I still find Last Week Tonight mildly entertaining. But I don't think he has anywhere near the chops to make his show must-view in the way Stewart did even during the Obama presidency. And I say that as somebody who still watches LWT every week, but finds himself caring less and less.