I love the aesthetics of the old MM rpgs, but... I've been finding it a tad difficult to get into them. After the tutorial for MM6, I just start feeling lost and directionless, not really sure how to pursue quests or where to go.
That said, which game is better suited to getting to know how this genre works, that provides a smoother experience to modern audiences?

I found M&M3 to be the most impressive of its time (worth looking into emulating MT-32 sound), but if you want 3D, I'd say M&M7 is quite easy to get into. It's still very similar to M&M6 but without the swarms of enemies, and travel is generally much easier.

They come in three different "styles". MM1 and 2 play similarly (you can load your MM1 party into the start of MM2). MM3, 4, and 5 all play similarly (same party can go through 4 and 5). MM6, 7, and 8 all play similarly.

7 might be the easiest of the modern set to get into (8 is the easiest with regards to difficulty). The story and areas are all a little more compact. Be prepared to do some exploring and talking to people to find things, though. Especially in 6, the quests give you direction on things to do, but they won't take you everywhere.

Of the middle games, I would say that MM4: Clouds of Xeen is probably the best entry point. MM3 can be a little rough at the start, particularly with enemies causing insanity, while MM4 doesn't have that issue. MM4, on the other hand, has a gentle start, has a competent default party (so you'll be fine even if you don't want to create your own characters; MM3's default party lacks a character to cast Walk on Water, which is a rather important spell in a world consisting of islands), and even offers an easier difficulty setting.

Furthermore, you don't have the somewhat archaic gameplay that the first two games have, and doesn't have the complexity of the skill point system that the later games have.

One point of advice: If you play World of Xeen, which includes both MM4 and MM5, and you want to fully enjoy MM4, don't use any pyramids until you've defeated Lord Xeen; otherwise you'll end up in MM5 and can easily earn enough XP there to make MM4 too easy.

(Also, ignore Swords of Xeen for now; that game is not for beginners, and it definitely has its issues.)

mm8 is the best starter, its the easiest and fastest of the games. its also way more forgiving and you can change your party mid way through so you won't get stuck with your creation that you didn't really know wtf you were doing with. another helpful thing is automatic map markers so you wont have to spend 20 years searching for teachers.