I'm not sure what your qualifications for "practical" are. I mean, I've seen some rather extravagant and inefficient kicks land on their target and knock people out. What circumstances are these to be used under, anyway?
The round kick is probably the most common kick in sporting competitions, and can be aimed virtually from head to toe. Aimed low (below, at, or a little above the knee), a powerful or well-timed kick can be used as a sweep. A head kick can end in a knockout, and I've seen fighters stopped by kicks to the body and thigh. In a street situation, a round kick to the groin, while probably not a fight-ender (it's a tougher target than people give credit for), can be used as part of a fight-ending combination. The main disadvantage of a round kick is that it's relatively easy to see coming, compared to some of the other kicks.
A thrusting front kick is not usually a knockout shot, but can be used to off-balance an opponent, keep them at a distance, or set up other striking combinations. Common targets for this kick include the knee/shin (a broken knee will probably NOT be the result), the hip/pelvis, the body and chest, and possibly the chin. Lower front thrust kicks can keep you in range for other strikes; the higher the kick is, the more room it needs, and the greater the possibility that it will move the opponent out of range.
The thrusting side kick is similar to the thrusting front kick, but generally has more range and power. Because of this, it's harder to use in combinations, but can be more damaging and keep the opponent farther away. Bruce Lee used this kick often as a "stop kick". Its greatest probable damage is breaking the ribs of an opponent.
The spinning back kick can theoretically be aimed anywhere, but is most often aimed at the body, and has caused sport fights to be stopped due to the pain and damage of such a shot.
Spinning wheel (hook) kicks don't show up too much in sport fights, and based on Kyokushin and Taekwondo competitions, seem prone to compromising the kicker's balance. However, they can cause a knockout if aimed for the head, and the late K-1 fighter Andy Hug would throw them to the knee/leg of an opponent and often did notable damage.
The oblique/inverted/cross-stomp kick has a limited range, but when aimed at the knee or shin, can stop an opponent's forward momentum, and even damage the leg. Its close proximity means that it can theoretically be used in conjunction with other strikes.
A snapping front kick, making contact primarily with the instep or shin, can be aimed at the groin or chin of a standing opponent, but can also be used as a "soccer kick" against a grounded opponent. Soccer kicks frequently result in knockout, though in a street situation, it could be perceived as excessive force.
I've never seen a stiff-legged "crecent kick" land with any kind of beneficial effect. It's extremely telegraphic. I've seen some kicks called "crecent kicks" utilize a snap of the knee; those seem to be effective to a degree. One is similar to JKD's "inverted hook (round) kick", and while I've never seen it cause a KO, it seems a decent distraction technique. Another, often found in hardcore karate systems, resembles a cross between a front and round kick. MMA fighter and Kyokushin practitioner Katsunori Kikuno frequently uses this version in his fights, hitting with the ball of the foot to the body.
These are the kicks I'm most familiar with. There are many, many more, most others being jumping or spinning variations of the ones I've listed. For either sport (MMA) or self-defense purposes, I would focus on hand techniques and have knee strikes be the most-trained leg attack. Following that, the basic round and thrusting front and side kicks would be more than enough for most people. Kicks are a support tool in most cases. The room needed to use them hardly exists in a self-defense situation, and their use leaves one on one leg with far less balance than on two. Take it from a guy who's a pretty good kicker: stick to punches/hand strikes for practical usage. Study the kicks to round yourself out and understand them, but spend about 80% of your striking training on upper-body tools.