Should they be housed alone, in pairs or in groups?
All hamster species do well by themselves. Some hamsters can live in pairs or groups, including Roborovski, Campbell, Winter White and Hybrid hamsters. Syrian hamsters are solitary animals, so should always be kept this way in pet ownership. Chinese hamsters generally do best when housed alone as they are also solitary animals - some people do have success housing them together, however I would personally be inclined to house them alone due to the risks of disputes.
If you do take on a pair or group of hamsters, you must ensure that the hamster species are compatible living in pairs/groups. You must also be prepared to separate the hamsters if fighting gets serious. From past experience, I would recommend getting just one hamster for new owners.
Even with a large home, being litter mates and having two of everything (including wheels, toys, hides etc) fighting may still occur. In some scenarios they may get on well for a while, then after a few months may start showing territorial behaviour (especially during adolescence) and as a result may have to be separated. Having pairs or groups is definitely lots of fun as they are really entertaining when interacting with one another, but you must be willing to separate the hamsters if this territorial behaviour is putting them at risk.
The following text is based on my own experiences and research.
Which type of hamster makes the best pet?
Of course there are different opinions on this subject. Each hamster species is very unique, as well as their individual personalities. For instance, Syrian hamsters are generally the easiest to handle and tame, but each hamster is very different, so you can't guarantee that all Syrian hamsters will be. Different hamster species will suit different people for different reasons.
Here is a list of the most common domesticated hamster species:
Campbells Russian Dwarf
Syrian hamsters make great pets, especially for those who would like to experience pet ownership, but not not yet prepared for the responsibility of, for example rabbits and guinea pigs. This hamster species are generally the easiest to handle as they are a bigger size than other domesticated hamster species. They are relatively easy to tame, and are especially good for first time hamster owners as they don't move around so quickly so handling them is considered easier with Syrians. Other dwarf species may be more entertaining to watch as they tend to be much more energetic and faster than Syrian hamsters, but they are still great fun to watch! Syrians do easily escape from hamster playpens, so you want to ensure that you purchase a playpen which will be hamster safe. They require a spacious habitat, and those sold in pet shops are almost always unsuitable. As well as a large home, they do also benefit from larger habitat accessories including an 11" wheel, and the hide houses, toys etc generally need to be bigger than what other hamster species require. Syrian hamsters tend to have a more calm temperament.
Roborovski hamsters move so quickly and are very small, therefore I would not recommend these as a first time pet for children. Robos are definitely a good option if you prefer a hamster to observe rather than handle as they are so entertaining, although this of course is dependent on their individual personalities, and some may be very tolerant of being handled. They still require a spacious home, despite being smaller than the other hamster species, though you may get away with a slightly smaller wheel and smaller cage accessories. They are certainly very sweet little creatures and I love how they are so tiny! They can live in pairs or alone, although be prepared to separate them if you get a pair or group of roborovskis as they may fight. They do love exploring so providing an enriching setup is important!
Most pet shop hamsters labelled 'Russian' are often considered to be Hybrids. Campbell Russian Dwarfs & Winter Whites are two different hamster species which have been inbred and as a result we now have Hybrid hamsters. Russian hamsters have striking markings and may even change colour (the images below are all the same hamster!). Similar to robo hamsters, Russian hamsters may be housed either solitary or in pairs (even though they do not need companionship) but be prepared to separate them if they start fighting. They can be handled, and are generally easier to hold than roborovskis (this is because they are slightly bigger and don't move around as fast). It is important with any species to respect their temperaments, so if they feel threatened or uncomfortable by being handled, then you should not cause unnecessary stress. Sometimes it is a good idea to take things slow with the taming process, so don't rush into the whole handling thing. It is important to remember that if you would like a hamster, not to just expect a cuddly pet - for sure it is very rewarding when they gain your trust, but also be respectful and take things at your hamsters pace. Some hamsters may never become as tame as the owner wishes and can bite, yet there are still other ways you can enjoy interaction, such as hand feeding your hamster.