A good tripod head is as important as the tripod itself. Besides specialized heads like panorama-heads there are mainly two types of tripod heads: 3-way heads and ball heads. I use both types for different needs. Right now I only use heads from Manfrotto, equipped with the proprietary hexagonal quick-release plates from their "RC0" system.
Ball head: Manfrotto 488 RC0
Weight: 755g, height: 13,4cm
The 488 ball head is available with different quick release mechanisms or with a simple 3/8" screw. After using the RC2 quick release system for two years I recently settled for the more sturdier RC0 system with the hexagonal plates.
I use this 488RC0 head mostly on my monopods or on the 055ProB tripod when I want to travel light. The head features a separate panorama-lock and a friction control/lock wheel. It is possible to reduce the friction very nicely with the wheel, which makes this head ideal for the usage on a monopod. A bubble level and a safety mechanism against unlocking the hex-plate is integrated into the quick release clamp.
The load rating of 8kg is a bit optimistic as usual. I would not put more than 6kg on it, which should be more than enough for most lenses. Ball movement is smooth and the friction control does a very nice job. Overall the ball head is a joy to use and a good improvement over the 486RC2 that I used before.
Ball head: Manfrotto 268 (RC0)
Weight: 1535g, height from base to ground level of the camera mounting plate: 15,6cm, total 16cm
This is a truly massive head. I think it is the biggest and heaviest ball head that Manfrotto ever produced. Right now Manfrotto has no ball head in the lineup that is as big and heavy as this one. It is intended for large format cameras and almost seems to be overkill for 35mm cameras. It is rated for 12kg load and this head should take the load without any problems.
The head features two levels at the side of the quick release mount, a panorama-lock and a friction control. The ball is locked through the big lever on the right, which can be turned up or down to lock the ball. The horizontal position sets the ball free, whereas this "free"-setting already has enough friction to hold a small to mid-sized camera/lens combination in place.
Operation of this head is good overall but there are some problems. As soon as the ball moves it does so without problems. But due to the relatively high friction extremely slow movement causes the ball to stick. As soon as you settle the ball to a certain position it becomes sticky in the current position. It is not possible to move the ball out of the current position without applying a higher amount of force. This leads to a slight bump when you start a new ball movement. In reality this is fortunately a minor problem, small movements in order to position the camera precisely are possible, it is just not as smooth as with more modern ball heads. If you are looking for a very stable ball head that can take high load this might be the best option for people on a budget.
Ball head: Manfrotto 168 (RC0)
Weight: 760g, height from base to ground level of the camera mounting plate: 13,2cm, total 13,8cm
The smaller brother of the 268 ball head is quite large, too. Unfortunately the size does not translate in a faultless performance. The version pictured here is the older one without a safety lock for the quick release which is the only addition on the newer version, which features a black ball instead of the shiny metal.
Movement is not as smooth as with the 268 but the ball head has not the problem to become sticky once settled. Due to the relative low volume of the ball itself the head tends to change position (mostly in a downward direction) once locked. This phenomenon was long time associated with ball heads but is not an issue any more with more modern constructions. The levers to lock down the ball head are big and offer a firm grip, however I would not put more than a maximum load of 4 kg on the head.
3-way: Manfrotto 029 (RC0)
Weight: 1430g, height from base to ground level of the camera mounting plate: 15,6cm, 16,7mm total
This ia an older version of the 029 three-way head which is discontinued for quite a while. It is the smaller brother of the still available 229 head which forms the top of the product line (non-geared 3-way heads). The 029 head should be enough for any DSLR and even bigger lenses. The load rating of 7,5kg seems to be realistic.
The operation of the head is unproblematic, it is even possible to achieve different levels of friction with differing tension of the locking levers. The version that I use is the older one with hard-plastic grips and no safety-lock for the quick release plate. If you shop for a 029 make sure to get the modern version with shorter grips and safety-lock.
3-way: Manfrotto 141RC (RC2)
Weight: 950g, height: 13,4cm total
This 3-way head is already replaced by a version made of plastic. This older version - which was current until 2006 - is made of metal like all other Manfrotto heads.
The load rating of 6kg seems a bit optimistic once more. The levers tend to stick a bit once tightened. Other than that the head functions as one would expect.