Havent played Prime 3, but i thought the difficulty in Prime 1 was pretty hard but also a good level and quite satisfying--sort of like Galaxy if you try and get all the stars. Prime 2, on the other hand was too hard. The fact that you could be loose points if you're simply standing in the wrong place (in the dark dimension) is indicative of this, but some of the bosses were also just really damn hard.

No. Try playing the NES game for the first time. Thats a challenge.

Prime 2 is a bastard. Boost Ball boss, Emperor Ing, the last fight against Dark Samus, and the Maintenence Drone (some boss that relies on the Spider Ball, but not the Spider Ball boss) forced me to cheat the first time, and just completely bypass him to get to the item he was guarding, the Screw Attack.

Prime 1 and Prime 3 are tough enough the first time you play them, but afterwards become very easy.

the final dark samus fight in prime 2 is as easy if not easier then the first boss in prime 2. whats so hard about him?

I beat Metroid Prime on Hard Mode without any health/missile upgrades. It's generally not that difficult since you get health and missile drops all the time. The exception would be the Phazon Mines (no savepoints, Power Troopers, Invisible Drone fight, morphball puzzle, Omega Pirate), and the excruciatingly long (PAL) fight with Metroid Prime, where you die in two hits.

The other games I never bothered replaying.

I found Prime 2 the most difficult, when I replayed them early last year.
My first playthrough of Prime 1 was difficult, actually, but Prime 3 wasn't particularly tough.

I did all three 100% in one week and thoroughly enjoyed it, and like I said, 2 was most difficult. I'm sure if you stick with 3 you'll be ok. Just make sure you look for health and missile expansions.

What? Is the PAL version harder than other versions, then?

Yes, Retro made numerous changes to the game between the American and European release - among them was a new AI/attack pattern for the first form of Metroid Prime, as well as doubling the final form's health. Moreover, some rectifications were made to the plot. However, most of the changes were concerned with glitches and sequence breaks, making it impossible to (for example) get the High Jump Boots as the first item in the game. A lot of purists and speedrunners therefore don't like the PAL version, though I think it is probably better for normal play.

Hmm, I see. That would explain why I had so much difficulty with that final boss - he was HARD. Maybe they'll fix that in the trilogy, would seem like a good idea to me, I mean, if people wanted more challenge they could always play the game on higher difficulty settings.

Roffle, Prime 3 is the easiest. In terms of difficulty I would say it's Prime 2 > 1 > 3. If you want to throw Hunters in then I would put it between 2 and 1, it's more pure FPS and doesn't have a lot of the conveniences the console games have. And Primes are generally easier than the main series, if Other M is channeling their difficulty it will most likely be harder.