aramis erak
Legend
When I say "old school," I'm mostly thinking of the 70s to 80s, but at its broadest I can extend this to pre-2000s games.
Palladium Fantasy: Many people hate Palladium's engine. What it did was take the concept of percentile skills (from D&D OE Sup I: Greyhawk), the skill concept of them being mostly by class & level, the concept that 3/4 of attacks would hit if not actively parried or dodged, but most would hit armor, and the armor has a limited life before losing effectiveness. (for those familiar with later versions, this edition had neither personal SDC from skills nor MDC at all.) The attack roll is made, and a modified 5+ hits, a modified 5 to AR hits the armor, damaging it, instead of the wearer; an AR+1 or higher roll hits HP directly. (in later games, AR+1 or more hits pSDC if the character has it, so most combatant characters take the "full physical package" - 4+ skills which give pSDC)
The Atlantean Trilogy was essentially the same conceptual issues with the same D&D mechanics as Siembieda had in Palladium Fantasy, but better executed solutions across the board. It just had no real visibility. They assumed a 50% hit, and made armor damage reduction; HP growth is a fixed number per level. Skills for non-combat are base plus percent per level, combat are tables giving modifiers at specific levels. There is a new version of the mechanical book, The Arcanum. It's the same rules, less one race which was excluded from the new edition's license, cleaned up. One other thing to be aware of: Dual Class/Multi-Class has an entirely different meaning from the rest of the industry.
T&T is actually 3rd, by a few hours, per Ken St Andre. Another TSR one beat it to the punch... at the same convention. T&T A New Age is taking the MR concept, the SR concept, and the love of open ending, into a different, count successes, model. It's decent, and in play, felt very T&T to me. (Most of the naysayers I'm seeing haven't actually run it.) But it seems early in the cycle. In all editions, play is attribute driven; everything is either an attack or a saving roll. Excepting the 90's videogame and ANA, combat is usually done as one big total, then damage divided. The videogame changes this to individual stuff. ANA allows ganging up, and can be done the same "all one attack" but it's intended on individual vs individual as the core point; it's clear they were looking at the videogame as well as the alternate rules in 7.0 when building ANA.
TFT: The new edition makes a dozen or so changes, and at first glance, they're pretty minor. But really, they're a major overhaul. The biggest change is mostly cosmetic: 1E was metric; LE is English Traditional Measures.
Rolemaster is, once you learn it, actually a fun system. But character gen isn't quick. Yes, it is a bit table loving. If you run core rules only, no options, character gen takes experienced players 10-20 minutes, vs 20-60 for new-to-system players. The 3 most used tables are the Static Maneuver Table, the Moving Maneuver Table, and the Experience Guidelines table. The experience worksheet is worth laminating and using overhead pens on, as you fill it out every session — the game assumes players will pay enough attention to log their damage done, damage taken, crits done/taken, etc on the experience sheet during session. Combat, each weapon type has a 1 page table; Give players a copy of their weapon's table for faster play. Critical hits are insanely common, and there are several, but they're easy to use. Almost all non-combat actions use either the static or moving maneuvers table; GMs will memorize them soon enough.
HARP is a modernized streamlined RM light.
If you can find it, MERP (Middle Earth Role Playing) is an excellent general game; whether it's good as tolkien is a matter of much debate. But it's an excellent RM light.
There is a MERP pseudoclone: Against the Darkmaster (VsD). It makes a few changes, and is clearly someone's "MERP 3E" homebrew. It's pretty and well written.
AFF (Advanced Fighting Fantasy) is back in PDF, and fairly light. I've got it, read it, never run it.
Dragon Warriors: DW is a fairly oldschool design. It works, the most important thing is that it mechanically goes in different directions than AD&D did... rolls to hit are more successful, but then you have to penetrate the armor, doing a fixed sum of damage. It is brutal on low level characters. Moreover, the Elven Crystals campaign is super dangerous - take hirelings, you WILL need them as replacement PCs. The PDF version on DTRPG is a modern reprint with errata applied and a few small changes.
Talislanta: same publisher as The Arcanum. Moved to modifiers only on attributes, and some editions skills are modifiers to a d20 roll. Combat works similarly. And while there are indeed no Elves, there are a dozen "very different look, but elves under the skin" races. early editions are essentially an atlas of a region, and include the races for that region. Many users used it as a modifier set to D&D, adding the skills in from Talislanta. The "Big Blue Bullet Stopper" (Yeah, someone demonstrated that .223R and .30 won't go through. On youtube.) is a consolidation, and is fairly rules medium-light (2/5).
The RPG, in 1st ed, is not very wargamy; yes, it's the same setting, but the game is very much more than just battles, and the published adventures vary widely in tone. La Maisontal is a save the monastery from the ravening horde, and the intent is to play out the final battle in WFB, and including the PCs. Meanwhile, the Enemy Within campaign can be run with few combats, and a very "fantasy CoC" feel, but players often pick fights. Since the weapon damage systems are different, but scaled the same, I usually just used the PCs stats, with the damage mechanic of the target's system (WFB or WFRP), but there are formal conversions. It's in PDF on Drive Thru. Conversion of WFB 1 to 4 figure stats to the RPG is pretty easy, too. The combat rules are intending to use figures on the table, but that was common for many; better to have the rules, and let GM's ignore them, than lack them and have people reject the game as incomplete.One ttrpg I considered is Warhammer Fantasy, but from what I know of the franchise it's heavier on the wargame side than the RPG side, and I'm not that into wargames.
2nd ed is decent, and just as much able to be Fantasy CoC in tone. It's got some balance issues.
The 90's had the Arduin standalone game. Quite interesting. Never played nor ran it, read a rommate's copy.The original Arduin Grimoire volumes aren't a game in their own right. They don't say what they are, but they're a load of add-ons for early versions of D&D.