Thgoughout the 90s the game I played most often, in a number of memorable campaigns, was Rolemaster. It's a game with, among other things, a very interesting history. It began as a series of plug-in modules obviously intended to replace subsystems of D&D. These began with Arms Law and Claw Law in 1980, which together completely replace (with some work) the combat system onboard the D&D of that era. Spell Law, released the following year, did the same for the magic system but was a little harder to work in because it required modifying or replacing D&D's classes, and whose rules implied that you were supposed to use it along side the earlier products. With the publication of Character Law in 1982 it was possible to play a game that was all Rolemaster without any D&D. Several editions down the line you can still see the fingerprints of this initial design impetus.
Rolemaster has a reputation for being fearsomely complex and terribly clumsy, but by the time I settled in to playing it around 1992, the edition that was extant at that time (today called the second edition) played very smoothly if the people at the table knew what they were doing and separated the responsibilities of the table intelligently. Everybody tracked their own XP for the most part, and looked up their own attack and critical results. So for a given encounter no one person needed to have more than a couple of photocopied pages of tables in front of them. It's the tables, in fact, that give Rolemaster its mystique as a monster of complexity, because there are seemingly entire books filled with only them, or so it is said. This is not really true, but any given edition of Arms Law is going to be about 80% full-page tables. This is scary.
What's hard to see until you actually play Rolemaster, though, is that there are really only four tables that the entire game runs off of. There are the static action and moving maneuver tables, which govern non-fighting actions, and there are the attack table and the critical table. That's really it... just those four charts. There are many different versions of attack tables and critical tables, which is what fills up the books, but you only use the one for the weapon or attack type you are using; the numerical results are different on each table but they otherwise work the same. And there is a separate static action table for spellcasting and different critical tables, with the one you use depending again on your attack type. The rules for using all these tables are found in just a few pages. Leave out the charts and lists and Rolemaster is really a remarkable tight game: roll open-ended percentile, add your action bonus and whatever other modifiers and look the result up on a chart. There is no separate damage roll — it's on the chart — but there is a separate critical roll that comes up a fair amount. That's the whole game, outside of character creation.
I think Rolenaster does a remarkable number of very fun things at the table. The open-ended rolls and tongue-in-cheek critical tables keep a fight unpredictable; any roll could be somebody's last. Sharing the bookeeping responsibilities helps keep everyone engaged. The magic system is rich and expansive, and for its day Gamemaster Law had some crackerjack worldbuilding advice. In the early editons the skill system was nice and loose and groups could feel free to explore their own boundaries for each skill. But I think what I like most about Rolemaster from a design point of view is its thematic cohesion. Each of the classes has a specific place in a strongly implied cosmology, and each of the many different types of magic taps into one or more corners of that cosmology. Magic items are another extension of the way the Rolemaster universe works. It's very clean and consistent without being too confining.
Later editions, starting with 1995's third edition, called Rolemaster Standard Rules, brought many previously optional systems to the core game and rigidly defined every single corner of the game. Skills became tied to individual type of actions, so for example to be an effective herbalist you needed one skill to find magical herbs, another to prepare them and a third to apply them to your patients or victims. There are some things to like about RMSS but it's also very, very bloated in comparison to the earlier game system... something it shared with the third edition of another fantasy RPG.
The Inspirations series highlights RPGs having special influence on or inspiration for the design of An Age Undreamed Of, the Sundered Reach or the World of Ytherra.