For me it's not Master of Magic (never played it, but want to now after hearing how, basically, perfect it seems to be).
For me, it's Age of Wonders. Got all those games, loved them all - have been looking for another one ever since. Maybe even some Warlords 3.
Actually I think true, old-school, RPGs are just as rare. The Bard's Tale, Might and Magic types. Wizardry 8 was the last one to give me that feel.
MoM is far from perfect, but it has a unique feel. I think the graphics have held up well (16 years, guys! We're all old now...), and if you want to grab it, it's like $6 from gog.com. I highly recommend it, but you may want to google some screenshots first. There's none of this campaign nonsense. Significantly different gameplay based on Wizard design and difficulty, too. Warlord, Crusadered, adamantiated berzerkers... with flying. Mess you up.
If you haven't played it, but enjoy tactical combat/RPG(ish), you should check out XCOM:UFO Defense (the first, not the subsequent ones). Wizardry 8 was really a remarkable game, too. Another heir to a venerable series, it was a work of love, and had both depth and polish.
You know, now that I think about it, that "work of love" thing is probably what ties all these really great games together. I think my expectation of a Gold, release-ready Elemental must be due to the obvious dedication and integrity of Stardock and the dev team. When you have talent and passion, things happen that just work.