Hi giancarlo. Check my signature. There are two perfect and reliable hosting companies which have good reputations and long work history.
There is nothing like top i guess coz even small companies maybe good but if they are not popular not every1 would know
There are a lot of top companies that fill various niches. You have your mass market hosts great for certain people. You have others specializing in application hosting. Now if you're talking about strictly top as in the largest that answer is obvious bluehost, hostgator and dreamhost. If you're talking about dedicated or co-location then it's a little tougher to tell. Obviously ThePlanet is popular and very large and SoftLayer has a substantial presence as well.
There are so many out there. All depends on what are you looking for! Remember though, they all have business proposition, so do some research!
in my opinion the best company is running from your city for example if you live in India china Iran Pakistan etc and you host your site in US for $4/pm you face some errors in hosting twice a month would you like to spend more then $10 on call phone to contact company???
if you have doubts about web hosting you can try this (have a free plan): freewhosting.co.cc/ if you like the service you can change to a paid host plan
Each company has their pros and cons. Some serve certain markets while others serve all areas of hosting. For example, some companies specialize in VPS hosting while others will offer shared, reseller, VPS and dedicated servers. It all depends on what your requirements are, as not every host will be able to meet your needs and budget.