For Def Leppard, it's Joe Elliott. I got into them with High n Dry, and still wish (a little bit) that they'd kept the harder edges, but they didn't, and yet, I still buy every record, because I feel like they made the commitment. They didn't ditch Rick when he got hurt, they didn't ditch Steve when he was clearly taking his own road, they didn't ditch Viv when he got sick... and yet, they still tour arenas without an album, and when they do put out an album, it still moves a decent number of units (albeit not Hysteria numbers). I dunno; we talk a lot here about "in a perfect world, Stephen Wilson (or insert your own obscure prog musician) would be where Taylor Swift is" and I personally think that's a bullshit argument, but I think Def Leppard is that case. Good guys, decent human beings, and they hit it big without too many compromises (Pyromania and Hysteria are poppier, but I don't view them as "compromises"; in fact, they were pretty big risks at the time).
I also got into them with High n Dry. Still a favorite of mine. Pyromania was my first concert! Though I saw the Hysteria tour a number of times, I thought that album was terrible. I've given them a number of chances over the years, but their albums are unlistenable to me.
Though I will say I made it through their most recent album on Spotify without puking.
For whatever reason, I loved Hysteria. I had all the 7" singles, with the excellent b-sides, and carried around a cassette tape that I later dubbed into .mp3 so I could listen in the car (they were finally released on the expanded Hysteria). Some of the recent stuff, well, there's always one or two songs that are a little too... radio for me, but I like pop music, and they write good melodies, so there is always a song or two that surprises.
It wasn't my first ever, but probably one of the first, I saw them OPEN for Billy Squier, and as good as Squier was (I'm a big fan of him as well) Leppard blew them off the stage. It wasn't even close, in my opinion.