Of the aforementioned, playing mostly Classic, having been in this game since open beta, having quit somewhere after Un’Goro’s launch, returned for Classic when I learned that it was a thing — partially due to nostalgia reasons and my belief that the game has mostly gone downhill, partially wishing to try the things I couldn’t back in the day — and subsequently stayed also for Mercenaries. No interest in Standard (let alone Wild) and yet another ‘My jaw that bite’ or whatever they come up with (is it Theotar nowadays? No matter), besides, the game has had a P2W reputation since some point, unlike what it used to be.
I wouldn’t say that were much the case for Classic. There are maybe a couple of legendaries plus a few niche ones that you really need (and no, that doesn’t even include Ragnaros; Leeroy and maybe Thalnos are more or less ‘must-have’, some situational ones like Mukla or Cenarius could be used, but are not even that necessary, and that’s about it), and there are cheap (i.e. without any legendary or even epic cards) decks out there you could play competitively. Some of the more expensive decks or cards are fun, but overrated.
That’s quite a poor and superficial analysis of the game mode. Besides, I’m playing more or less none of these stupid decks (well, still running just one combo in my ‘big taunt’ druid and laughing at those no-brain netdeckers; played a bit of freeze mage, but it’s actually quite bad and weak there, being more or less a counter to some obnoxious decks, but having a hopeless match-up against a number of others) and having great fun (well, and otherwise in terms of emotions, to be fair, but that’s another matter) and success.
To sum up, I would probably recommend only Classic, but mostly if you are nostalgic or something like that, although it’s still somewhat good.