Apologies for the late reply to @PaisleyUnderground.
The request was to provide 3 or 4 bands that might intro the Stoner, Sludge, Doom genre(s) to new listeners.
It’s easier to provide 3 or 4 bands per genre, given how diverse each genre/style is. I’ve covered Stoner earlier in the thread, so, below I’ll try to ease-you-in to the Doom sound.
Doom, in it’s purest form, spawned directly from Black Sabbath, Pentagram and to a lesser degree bands like Sir Lord Baltimore et al. Arguably more than Stoner or Sludge, Doom has produced a multitude of sub-genres and stylistic variations, as @pyrexia alluded to by reference to death-doom. Some of the sub-genres (actually, stylistic variants) are known as the following: death-doom, blackened-doom, funeral-doom, drone-doom, doom-gothic, progressive-doom and many others. Outside of metal, such variations as doom-jazz also exist. There are also a good number of recognizable regional variants e.g. Finnish Doom, Louisiana Doom, Pacific Northwest etc., all of which incorporate slightly different stylistic elements but have become distinct in their own ways.
Bands such as Cathedral, Candlemass, Trouble, Saint Vitus, Pentagram were the true early pioneers of the genre. A complete and comprehensive list would, of course, be painfully long.
Historical significance is a given for Black Sabbath in terms of doom, but for new listeners they may be unaware of how much Pentagram contributed as they have never gained the kind of mainstream or cultural recognition, most people have heard of Black Sabbath not so much with Pentagram.
I posted this album earlier in the thread, but it has key significance to understand the roots of doom:
I would then fast forward 14 years to Sleep’s Dopesmoker:
I would also take stock of one of the early classics of the genre, doomy as all hell from the mighty Cathedral:
Before digesting one of my favourite doom bands, Electric Wizard:
Whether Come My Fanatics (above) is better than Dopethrone (EW’s 3rd album) is argued passionately among doom fans - for me CMF just has it in terms of sheer intensity, but it’s a little like arguing as to whether a Ferrari is better than a Lambo … they’re both phenomenal works of art and engineering and both are deservedly iconic.
Doom takes many forms these days; sometimes brutal, sometimes low and slow but it is without question one of the richest and most diverse (sub) genres in metal, in my opinion.
For several years now my hands down favourite doom band has been Windhand - they have their sound so dialed-in; perfect speed, heavy af, exceptional musicianship, production etc. and those female vocals just speak to me. These guys are nearly always on my playlist somewhere. A little too overproduced for some, perhaps, but for me it’s a perfect realization and testament to the diversity of the genre.
Hopefully that provides some semblance of a balanced, albeit quick and dirty, intro to doom. Those looking for inspiration will find many resources online, some quick resources that are worth keeping an eye on are:
https://www.angrymetalguy.com/tag/doom-metal/