While all three systems are biased toward realism and therefore work best with relatively "normal" humans (as opposed to, say, superheroes), they span a quite wide part of the hypothetical complexity scale. In general, CORPS is quite simple while TimeLords is quite complex, and GURPS is somewhere in between (depending on just how many of the optional rules are in use at any time).
The main advantage of CORPS is its extremely streamlined rules. The central mechanic of this system features matching one's skill (or stat, or whatever) rating versus the difficulty rating of whatever one is attempting to do. If the task is simple enough compared to your skill level, you automatically succeed; if it is too much harder, you automatically fail; only if it falls in between "easy" and "way too hard" do you need to roll a die. The system is built on a scale where 4 or 5 is an average stat for a human, and 10 is the maximum, and skills tend to have similar ratings (skills above 10 are rare in a "regular" world); so all the math you have to do very often is add single-digit numbers (or numbers in the low teens). All this means that CORPS is a system which really moves once you've learned how it works (which doesn't take long); in my view, its main strength is the incredible speed at which its quite reasonable results are generated. Combat with modern firearms, for instance, should be a fast and furious and deadly thing; with this system, it is. Unfortunately, to get this kind of speed some detail must be sacrificed. The system's main weakness, in my opinion, is that it's relatively grainy when it comes to stat and skill levels; the difference between a 5 stat and a 6 stat is a rather large one, and there's no intermediary. Similarly, character advancement takes a very long time, since the "jump" from each level of skill to the next represents a large amount of learning (and makes a large difference in practical use, too). Also, I think the metarules for designing paranormal powers are somewhat fishy and vaguely defined (I still can't quite understand them), but at least they don't take up that many pages.
The main advantage of GURPS is that it's extremely well-endowed, um, with supplements and optional rules and source material, that is. And that it's more detailed than CORPS but less complicated, in its basic rules, than TimeLords. The main disadvantages with GURPS compared to the other two systems are somewhat vague -- it's more complicated than CORPS but less detailed than TimeLords. Also, its rules for injuries to characters, while workable enough as they are, leave something to be desired when compared to either of the other two systems. There is no real provision in basic GURPS for "non-lethal" damage -- punches are a bit too lethal, in my opinion, and while there does exist an official rule about the kind of hurt one might expect to take through flexible armor even if it isn't penetrated, it feels like a bit of a kludge to me. Also, and this is the real problem, GURPS' damage mechanics are designed around a "hit points" system, where damage accumulates until you collapse or even die just because you've been pinpricked one time too many. I've been working on a set of house rules to get rid of both these problems, heavily inspired by TimeLords I must admit.
TimeLords has a reputation for being horribly complex. This is only partly deserved; much of the reputed complexity is more grounded in poorly explained rules (especially in the earliest editions of the game) than in actual complexity. The centerpiece of the system is a set of combat/injury rules which IMHO are superior to the basic GURPS system -- it does not use hit points, but rather applies the effects of each injury separately. An injury results in the affected body part becoming impaired (anything from a slight minus in using it up to becoming totally disabled), and may also have various stunning/ unconsciousness results, and may or may not be eventually fatal (with "time to death" ranging from seconds to days, or in a rare few cases instantly fatal). The only thing that's cumulative is impairment. The system also distinguishes between lethal damage and non-lethal damage (which heals much faster and cannot kill you or cause permanent harm), and different weapons do different proportions of lethal/non-lethal damage (a gun or a sword is all lethal damage, while a normal fist is 75% non- lethal, for instance); armour has a dual effect in that it both stops some damage outright and converts some lethal damage to non-lethal damage, before letting anything through to cause lethal damage (bruising through armour -- typically, flexible armour allows much more bruising than rigid armour). CORPS follows much of the same design philosophy, but it is much simplified compared to TimeLords. The main disadvantage of TimeLords is that it is quite complicated, and a lot of players will be scared of it. I have tried running three different campaigns using TimeLords; one of them (the middle one, actually) failed because about half the players failed their IQ roll to understand the system (or maybe their Fright check on first seeing the system), but the players in the other two campaigns had no problems with that. The other disadvantage of TimeLords is that the rest of the system is not nearly as well-developed as for instance GURPS. Also, I disagree a bit with the central idea of the skill resolution system, which uses a linear d20 roll, with proportional modifiers for difficulty, in 5% increments of original skill -- it's quite well executed, but I have design- philosophical problems with it. I think I prefer either the bell-curve design of GURPS or the skill/difficulty design of CORPS.
In the "final" analysis, it seems that I cannot ever quite make up my mind as to which system is actually The Greatest. Generally, I choose to use CORPS when I want a mechanically simple and fast-moving game, but when I want more detail I can easily go with either GURPS or TimeLords. Regardless of whether I use GURPS or TimeLords, I always want to introduce modifications to make the system I'm using more like the other one; in the case of GURPS I'm currently working on a set of house rules to make its injury rules somewhat more similar to those of TimeLords; in the case of TimeLords I'm always trying to adapt all sorts of GURPS things (such as the magic systems from GURPS Magic or GURPS Voodoo, or the martial arts rules, or the stats for vehicles designed with GURPS Vehicles, or the entire skill list from GURPS, etc). Who knows, I might settle on some compromise one of these days.