Hi,
is there any difference if I want to buy bitcoins, from which site to do so? Is there a list sorted where trusted sites are on top?
There is a pretty big difference among all the sites, other than the obvious question of trustworthiness like the others have already mentioned.
There is the problem of fees. Some bitcoin purchase sites require id verification for anti-fraud precautions. There are also peer 2 peer exchanges like LocalBitcoins, the same goes in that the fees might be higher if you are buying smaller amounts.
If you do not mind verification, I would honestly suggest Coinbase despite their fees.