I have played with all those 4 sites and it would be hard to call one site as the best one as all of them are good to try and stay with them. If you are planning to have continuous gambling then I like to suggest to have accounts in all these sites and gamble in rotation.

freebitco.in is basically a faucet site but the most famous among dice gamblers as it has all the functionalities of automated dicing. You may get free BTC to try your strategies along testing your luck factors. Primedice is the old one but I did not play with them at least in last six months as I was busy in playing with other sites.

Bitsler and crypto-games are known for their reputation and problem free way of gambling due to their perfect customer support. Both of these sites are having options to gamble in many altcoins which may give you an extra edge to find same level of dicing entertainment but for relatively cheaper prices.

After all these 4 sites, I like to suggest you to give a try yobit.net's dicing. They are basically a crypto exchange but offering lots of altcoins based dicing. A must try if you like freebitco.in  .