GURPS: The Toolbox
So it's been said, by me included, that GURPS is a toolbox. A big hunking box of rules, consistent with each other even if not all meant to be used at once.
So what do I use?
For my current DF game, the important thing is gathering loot. To do that, the delvers need to rip through combats (especially lesser ones they can win with minimal/no cost) pretty quickly. At the same time, we want to preserve the balance and the excitement of GURPS combat - its potential lethality if you screw up with even a high powered delver.
We use elements from a lot of books:
The Basic Set - Characters and Campaigns, obviously.
Most of the DF range - 1-3, obviously, and 11, but not the Clerics book, because I just want fairly basic priests, and not so much from the Summoners book, because I don't want Ally Horde PCs in this particular game. I don't even use Ninja, which I created, because they add a specific vibe I don't really want for in my dungeon right now. No Psi, right now, but I make no promises about "no psi" in my games. Loadouts (DF 13) has been extremely useful for quickie NPC equipping.
GURPS Martial Arts - I co-authored this. So obviously I use the living hell out of the whole book, right? Well, if you've known me a while this won't surprise you - I only use a select handful of rules from it for my DF game. The expanded Feint rules (Beats, Ruses, defensive Feinting), Telegraphic Attacks, and the new maneuvers like Committed Attack and Defensive Attack, of course. The multi-shot rules for missile weapons for Heroic Archers and Throwing Art masters. The improved defenses for two-handed weapons, too, and multiple blocks. Most of the other rules are either aimed at a more detailed, simulationist game (like A Matter of Inches, or the Harsh Realism rules) or use rules that would complicate character generation (martial arts styles, say, or leveled Techniques).
GURPS Low-Tech - another co-authoring job. All I use from this are a couple of bows, the weapon customization rules (sometimes), the weapon and armor scaling rules from LTC2, and the "new" damage for unbalanced two-handed weapons and shortswords. Sometimes some of the equipment. It's great stuff, and the armor is exceedingly well researched, but it's far, far easier to use the simplified stuff in Basic Set.
GURPS Magic - got to have spells. I limited the selection a bit more than DF does, though, because there are spells I just find frustrating in actual play after decades of using them. Bye bye Bless! You've been forgotten until too late too often ("Oh, wait, I had Bless from last session. I forgot I had a +1 on all my rolls . . . ")
GURPS Powers - sometimes. Mostly for statting up monsters properly so I can potentially publish them after playtesting them. AKA after my players chop them up or run in terror from them.
So, dozens of 4e books, and I use that handful, and not everything in each one.
Here are some of the rules we choose to use.
We've been using the basic combat rules, with those GURPS Martial Arts options I mentioned above, for our "easy" or "unimportant" fights - the ones the PCs should win easily, or which are too big for complexity. We've simplified them a bit further where we could, again, especially where the PCs would have an overwhelming advantage anyway. We also use the advanced combat rules for really nasty fights, the ones that are especially dangerous and/or cool. Anywhere a special rule leverages something a PC has - extra movement, a special weapon like a pick, etc. - we use it. Otherwise, we've been picking rules based on "Does this make the game go faster without comprising the fun?" - if the answer is yes, we use that rule.
Plus, mooks/fodder types fail all HT rolls for consciousness at 0 HP or less. Major NPCs, leader types, elite troopers, etc. generally get rolls. One exception - if a "mook" does something cool, I might extend them the courtesy of rolling their HT checks. Maybe they might make a real fight out of it.
Otherwise, we don't use a lot of optional combat rules, even the ones we used to use in our last game. Back then, we'd have a lot of fights, but one big fight for a session (or even a 2-3 session epic fight - we had at least two of those). A lost fight would derail the plans of the PCs after years of progress, so we took our time and turned all the options on. In my DF game, a lost fight means retreat and loss of treasure and maybe whipping up some new PCs - beer and pretzels instead of a serious "campaign." Different rules for different play styles, even with (largely) the same group.
Otherwise, we use the basic rules for everything else. Not a lot of options, and none of the real complicated detail rules more suited to a simulation or more serious game. Some I'd like to use, but my players don't really care to do the work for - tracking FP for extra effort in combat, for example. We'd probably enjoy some of the extra detailed grappling rules Douglas Cole has in his upcoming GURPS Martial Arts: Technical Grappling, but they'd extend time on combats we'd prefer to shorten.
The nice thing is that they're there if I need them for a special situation. I've got a huge reservoir of really internally consistent rules that hang together well.
But I don't use them all by a long shot. I just picked out the tools that do the stuff I need.