In general the deluxe tow-bars as approved by the vehicle manufacturer will be the best. Often these will plug-in to the chassis rails where strong-points are located. With this type the cars ground clearance will not be affected. Makes such as Brink, Thule and Oris are just three names that come to mind.
Next comes the "budget quality" tow-bars. With this type the vehicle manufacturers strong-points are usually ignored and drilling is necessary to fit what is basically a bodge-up. The usual design is for a length of rectangular hollow steel to run east-west across the rear of the car. A long length of rectangular hollow steel is then run north-south. As this will run below the fuel tank and/or the spare wheel well the cars ground clearance will be reduced by about two inches. Furthermore the metal in a spare wheel well is barely adequate for attaching the north-south piece of steel. Nevertheless millions of tow-bars like these have been made even though they often ground on "sleeping policemen". With this type of tow-bar it is important to fit spring assisters, helper springs or both. Another problem which may arise is self-levelling shock absorbers that no longer self-level. In general the paint codes on the road springs will be a Trade Secret and trying to get "towing springs" can be a difficult uphill struggle especially if the car is out of production. Having just said that it is worth noting that at one time the Ford Granada had at least four different options for its rear springs.
The most tiresome type of tow-bar has to be the swan-neck with a welded-on stem. Apart from not being able to use the gadgets that two-bolt tow-bars use, the stem cannot be unbolted. If the car is used in city centres where parking is very tight the inability to remove the tow-ball can be a nuisance. Incidentally whilst the premium quality tow-bars are usually laser cut the budget ones are usually made from steel that has been guillotined and the holes will have been punched. These methods leave sharp edges. The other things to watch out for are MIG welding "thorns" that can cut one to the bone and weld spatter. Looking on the bright side it is possible to sand-off the sharp edges and "thorns" with a flap disc in a grinderette.
If one has a swan-neck tow-bar with a detachable stem it is possible to get a new stem made that will accept a two-bolt tow ball. OK the tow-bar manufacturers may not approve claiming that the aftermarket stem is not E approved but if the car is pre 1998 it does not have to be! With a suitable British Standard two-bolt to NATO adapter it is even possible to tow a Sankey army trailer. Unfortunately some councils will now refuse to accept rubbish from this size of trailer at the recycling centre.
In a nutshell watch out for ground clearance, soggy springs and non-self levelling Nivomatts. Owners of some Citroens will not have any of these problems of course.
On the bigger trailers there is a LOT to be said for using the American method of crossed heavy duty chains. The reason is that if a rear tyre on a four wheel trailer bursts the load on the tow-ball can become negative. If perchance one did not have three hands available when coupling-up, that forgotten 50p coin used to defeat the triple-lock hitch mechanism may bite-back! Phew!