Not sure if you have checked it out or not, but there is a section in the FAQs about [historians' views of other historians] (http://www.reddit.com/r/AskHistorians/wiki/faq#wiki_the_study_of_history). Not sure if that is of any help/interest, but that is all I can really contribute.
Among ancient historians, I would put Polybius on top, because he used sources close to the event he talked about and criticized other contemporary historians. Moreover his point of view of the roman conquest, is pro-roman but neither unconditionnal nor centered on the romans (like Livy for exemple). There is also a darker side as I am pretty sure that sometimes he choose his sources poorly only to establish links and to shows lessons in history. The description of the murder of the elder son of the seleucid king Antiochus III (preserved by Livy) is completely ridiculous and should be seen as a way to force the parrallel with a similar act by Philip V king of Macedonia, allied of Antiochus.
This a question that can yield a lot of different answers depending on who you ask. No classical source is fully accurate, but a good amount of them are rather so. What's importance to realise is that none of them can be taken as completely accurate, it all has to be judged and compared to other contemporary authors, archaeological evidence, personal gain and so on. So with that in mind, I can give you a list of sources that I've found to be generally accurate.
Good ones include:
Tacitus - as long as you remember that he writes as a senator.
Dio Cassius - the one I personally trust the most.
Suetonius - same as with Tacitus, he has some personal goals with his writing.
Cicero - because we have letters from him that weren't meant to be read by the public, making less likely to have inaccurate propaganda in them.
Pliny the Younger - same as with Cicero.
Livius and Polybios, mentioned by /u/liwios, can definitiely be added to the list as well. They have their faults of course (Livius writes about the founding of Rome for instance, which, while interesting, can't be proven) and Polybios have a tendency of writing about the Punic Wars as if he were there (he fought in the 3rd Punic War, but not the 1st and 2nd).
I could go on with writing like this but perhaps you get the point? Many of our ancient authors are actually accurate if you put them in perspective, but can't be taken fully on their words; there are very few that generally shouldn't be used at all. One of those is Herodotos who, while he can be used for a sort of 'world view' perspective, isn't your best choice if you want a factual author.