Sometimes I wonder if these questions get divided up among multiple people - "OK you write days 1-20, you do 11-20, and I'll do 21-31" and if some of those people don't have much actual experience running or playing these games. At a minimum some of them have language issues - like this one! An RPG doesn't "provide" anything! Maybe they meant which RPG company provides the most useful resources? Maybe they meant which game has the best support? Would this mean only "official" resources or would third party or player-generated stuff count here too? I'm not sure but I'll see if I can come up with something coherent below.
The best resources to have from a GM point of view:
- A game you like - probably a book or a PDF
- At least 3 interested and available players
- A place to play
- A regular opening on the schedule
That's all I need to run a good game and I've been known to run without one or two of those things. I suspect this question is aimed at "gaming stuff" like splatbooks, screens, mini's, dice, etc. We can do better than that though, as I don't really think of that stuff as all that "useful" when running a game.
- A game with a solid forum where ideas and problems can be discussed has a leg up on others. Mutants and Masterminds, FFG Star Wars, and Pathfinder all benefit from this. I find a good forum particularly helpful with more mechanically complex games like PF and M&M.
- Paizo's Pathfinder forum is also extremely useful when running one of their adventure paths as a ton of player-generated material starts showing up as soon as the first volume is released - maps, player handouts, plot outlines, character portraits - all kinds of good stuff for actually running a game shows up in these. FFG's Star Wars forums provide a similar service for their adventures too.
- Green Ronin's M&M forum is a treasure trove of famous character writeups from well-known superheroes to things like folk heroes, video games, and D&D conversions. You could probably run an entire Marvel Universe campaign in M&M 2nd or 3rd edition without ever needing to write up your own stats. That's some useful material.
- Something I need to run a lot of games is NPC and monster/enemy stats in an easily usable form. It's not something that is always available.
- D&D 4E had the online monster maker which meant I could pull up any monster they had released in an instant, and I could also modify it on the fly with a few clicks of the mouse - that was incredibly useful for that game.
- FFG Star Wars makes 3 decks of cards that have complete stats and gear and some notes on different types of NPCs from stormtroopers to criminals to rebel officers. I suspect I could run an entire campaign using these and nothing else for friends and foes alike. They are extremely useful, much like the templates from d6 Star Wars.
- Pathfinder came out with some books - the NPC Codex and the Monster Codex - that have pregenerated stats and gear for character types and monster types at every level of the game. That's a start but having them locked in a big hardback book doesn't really help me during the game because I probably need more than one of them at a time. HeroLab to the rescue! If you pick up the add-on for those books for HeroLab you can pull up whatever characters or monsters you need and save them as an "encounter" within HL. That way you can set it up and save it ahead of time or you can do it on the fly and wing it as you see fit. It's an odd fusion of formats as the codexes provide a big bunch of NPC data, but it takes HeroLab to make it useful in my opinion.
- M&M has been very good at providing these from the Instant Superheroes book of second edition to utilizing HeroLab as well. Many of their books have a HeroLab support option.
- An aid to help players get up and running is another useful resource and Goodman Games' Dungeon Crawl Classics has a few of them:
- Purple Sorcerer has a handy zero level character generator and more.
- DCC also ran a kickstarter for scratch-off character cards which is another interesting way to get started.
- Another card option: condition cards. I think Pathfinder's official set was the first one I saw but there are player-created versions for M&M and D&D 5E and I notice they are one of the first scheduled products for Starfinder. Any game that uses something like "conditions" as a mechanic usually has a whole set of them and no one memorizes what all of them do. This is a simple, inexpensive, easily-implemented way to solve that.
- HeroLab deserves a mention of its own - beyond just generating characters it has tools for running encounters during a game. The more stat-heavy your game of choice, the more useful this is.
- The Pathfinder Combat Manager is something I've discussed before but if you are running that game and haven't checked it out you should probably take a look. It's what makes that game usable to me in spite of the detail and size of the Pathfinder Mechanics Universe.
So looking at all of that I'd say "useful resources" for me covers things that make actual play of the game easier, and that more complex games benefit from these kinds of things even more. Books that add more features for player characters are not really "useful" to me as a GM. Supplements that add more sub-systems to a game are not necessarily useful in that sense either. The Items I have listed above are the things I have found that make starting a game and/or keeping a game moving that much easier and so are quite "useful". To sum up:
- Resources where I can discuss or help prep outside of game time are useful for any game but are especially helpful with the more complicated games.
- Online systems or resources I can use for prep or during the actual game are really nice, especially for more complex games.
- Cards are a really useful thing even for simpler games. I like having NPC info or monster villian info right there, I like having rules tidbits like conditions easily at hand, and I like being able to toss someone a card with say an important piece of gear on it. The cards serve as both a reminder of the mechanics and an indicator of who has the item or who is suffering from the condition. I'd like to see more card type products for RPGs anywhere it makes sense.