I have never played an RM or MERP* campaign that lasted more than 6-10 sessions, so I can't mention anything from a game memory - but I sure have poured over the various magic item books and drooled. (The below are by no means exhaustive lists.)
RM (FRP/SS):
Loremaster's Amulet. A powerful item that scales a bit with the character. Has lots a lot cool spells and stuff, sort of like a mage's staff from D&D only less focused on blasting stuff.
Turang. One of the swords of the high elves, a magical longsword with big bonus's (that get 50% bigger vs demons and evil demi-gods - I guess they just couldn't bring themselves to even suggest to players that it might be possible to kill a full god...) and some cool power. Won't let no poser use it either.
The Spear of Light. Just an awesome good-aligned artifact that could easily be the major item of a sect of knights for a kingdom. The leader of which gets the honor of wielding the fabulous weapon.
Staff of Aristarchus. This one makes a great evil macguffin. The PCs have to try to keep it out of the [insert your big-bad here], and then have to figure out how to deal with - as they are likely unable to destroy it.
Plaguestone. Could be the cause of a deadly outbreak. (Though you may want to wait a while before starting that particular story line.
) But, the bad guys could be doing it to weaken a kingdom so that invading is easier - or a rival to the throne uses it to weaken support for the other contender(s).
Staff of Light’s Echoes. Just an awesome staff for channeler's and essence users (sorry mentalists, do not pass go... yeah, you get it). Big PP multiplier with some cool castable spells.
MERP (A note and a question, note first: I won't choose any that are said to be destroyed, just 'cuz, question: what is up with the weird bonuses in this book? I mean +88? What the heck? Couldn't be bothered to add 2?
:
Ainacrist. Big bonus, extra damage, no being stunned or fumbling. Heck, this one is worth it without the speed boosts it gives!
Ancaruin. Big (and weird) bonus and the added ability of being able to be summoned to your had from far away. (That distance is another weird number.)
Grond. Well, uuhh, err, lets just let Morgoth's hammer alone, hmmm? Yeah, just leave it in that deep, dark hole. Then pour the concrete. There you go. Now the 5 Olypic-sized swimming pools worth of Holy Water - you say that's just for vampires and demons?!? I don't care, pour it! I SAID POUR IT!!
Mace of Waterskipping. Just tons of imaginative fun to be had with this one! Skip it across water to hit someone and then it skips back? Yes, please.
Crown of Teleri. If you ever wanted to be Ororo (Storm, from the X-men), this will let you get close. Plus it makes you harder to hit.
Remmiraith. Well, if the 7 gems of this necklace, each one able to hold a spell that you can later cast instantaneously isn't enough to make you want it. Perhaps the fact that it is worth a literal king's ransom -
and then some - would help. (I think it is worth more than the Arkenstone, though that one doesn't have a monetary value attached to it and most of its value comes from cultural meaning.)
Late now, tired gotta go to bed. Good night.
*Hopefully the one I just started today on Roll20 will last. Fingers crossed.