The often overlooked first in the series may actually be the best.
This has a reputation as being one of the hardest games ever made. It even beat Battletoads on a Top 10 Hardest Games list. Is it this hard? No. My theory is that people just played the game in their younger years when they weren't as good and figured it was the hardest game ever then.
Is it hard? Yes. Guts Man will kill you all over the place when you start, and Ice Man manages to turn that up to 11 and make things a hundred times worse.
Yet I still prefer the simple style of this to the later games in the series. All the weapons have a specific use, unlike the godly Metal Blade in 2. Unlike 2, you will never need to grind killing enemies to restock ammo on a tool to progress. It feels more like an adventure. And the feeling that you get beating Dr. Wily is so unbelievably satisfying it's fantastic.
So yes, it is hard. Not as hard as they say, but still very difficult. But it's rewarding and enjoyable. So don't overlook it, it's worth the slog.