As a carpet cleaning professional I get asked this question quite often, to which i usually reply “As long as it has a power head and you enjoy using it that is probably the best vacuum cleaner for you.”
That said I do have a personal soft spot for the Kirby vacuum cleaner because I like to buy things that last and that do the best job, are there down sides to the Kirby? yes, you have to buy bags and is is very heavy if you are lifting it around, (but due to the direct drive system it is not heavy to push when you are actually vacuuming.) to buy one new is very expensive and requires you to engage in pushy door to door sales which is never the best way to buy anything. They can however be found on the second hand market for a steal and I highly recommend going that way if a Kirby interests you. Also because there is parts easily available should something go wrong with it they are easily repairable.
The thing you have to take into consideration with vacuum cleaners such as a shark or Dyson which are completely fine and do a good job. Is they are disposable if something goes wrong with them you are more than likely looking at buying a new vacuum cleaner. as no vacuum cleaner repair shops will work on them.
But they do have plenty of positive things going for them as well like not having to buy vacuum bags, you also don’t have to replace the drive belt for the brush role like you do on the Kirby (around every 20 uses) though you do have to make sure you keep the filter clean on your Shark or Dyson i would be doing this at least every month under normal household use. if i were to choose between shark or Dyson i would choose shark as they are generally less expensive and they suck just as hard.
A 4th option would be a Lux (formerly Electrolux) vacuum i would have rated these up with the Kirby vacuum cleaner but unfortunately Lux has stopped producing vacuums and it is getting harder to find parts these days they do a great job but for this reason i find them hard to recommend unless you are picking them up cheap and fully working.