Which reliable POS hardware do you recommend for a busy supermarket?

Good question.

Hardware is not the issue as most Pos Hardware can be service on site. Make sure your vendor has the resource and replacement parts to meet your SLA. 

waiting3 week for parts is not acceptable.

 

This is a very common question, but is a very IMPORTANT question and needs to be answered keeping in mind several factors.
The reliability of a POS hardware depends on various factors: 1.
The design of your check out counter will define how the POS system will be configured on the counter.
Universally, the process is scan --> total --> membership / loyalty --> Payment --> Settle --> Check out.
Since POS hardware is available in integrated (all in one) and distributed (all modules are seperated) forms your counter design will help you to ensure you have the right hardware.
Mostly, Supermarkets are found to use a distributed model allowing them to ensure that the system enhances operations and facilitates customer interaction which essentially ensures customers retention. 
The counter design will also help you to determine whether you can afford a15" screen or12" screen for operator, as well as a tall / short standing pole display or in built pole display for customer viewing.
The customer pole display also has various characteristics such as LED, LCD and VFD it is necessary to know the ambience and chose the correct display, Displays are also available with2 line20 CHR and4 line20 CHR as well as10" LCD displays. 
Again the selection of displays requires one to know what is going to be displayed.
is it only the rate and amount or will it be used for commercial purposes also.
Most super market owners treat POS counters as an expense.
Which is not the fact, a POS system can be used for promotions and as a point of information also.
running in store promotional ads and product display advertisements or future events allows for upsale or ensuring customer retention.
This functionality can also be used as a revenue earning model for supermarkets.
It is also necessary to know from the application point of view, what port connectivity is required for the various peripherals, whether serial, parallel or USB.
Hence one would need a scanner, there are different types of scanners handheld laser, table mount vertical image and table mount embedded image scanners.
It is neccessary to have at least one of the table mounts as well as a handheld scanner which allows you to scan labels of large size goods.
The other important factor one must know is the various SKUs in stock as this will ensure what bar code symbologies are required to be scanned and this will help determine the bar code scanner model / brand that you can opt for.
2. 
 Expected transactions per minute to ensure faster check outs.
This will help you to determine the speed of the processor / capacity of memory / hard disk space required.
Most software solutions work on a client server network, hence it is recommended to use a system with a better processor and higher memory capacity.
also it is no point having a system with4 GB of RAM if you are going to use a WINDOWS XP based application as it is considered a waste of investment as WINDOWS XP will recognise only3 GB of memory only.
Serviceability - The POS system being distributed also allows for better serviceability, this means if any part fails one can simply replace it and continue working.
Serviceability also ensure maximum uptime of the system.
Parts availability and vendor strength in the region: it is not necessary that bigger brands is always a better choice.
it is best to evaluate every factor above and also to understand the strength of the vendor in terms of product life cycle sustainability, high availability of spare parts.
support team strength are some of the key parameters.
Lastly, one also needs to understand the cost of maintaining the hardware, what are the warranty features offered and how they are most beneficial to you.
It is no point purchasing a POS system with a three year warranty when the product has been declared end of life3 years ago.
So to make you sure you are buying a RELIABLE POS System one must go through a thorough check of all above parameters.

Toshiba, IBM and Wincor. They are really designed for retail company and in food group. 

Look for  a company that has a very strong after-sales support team.