Great Work of a Classical Historian
Tacitus is one of the greatest historians working in Ancient Rome. His writing style and treatment of source material make him both an entertaining and effective historian.
Tacitus is a rare breed among the historians of Ancient Rome. He seems to be one of the first people in antiquity to discover that concise writing is far superior to flowery, heavily embellished prose when writing history. His clear, concise writing style is a model of brevity. His no-frills prose makes him easier to read and comprehend. His matter-of-fact tone lends his writing an air of authority and plausibility. He is immanently trustworthy. We still need to be critical of his works, but he has critiqued both his own work, and his sources, which makes it easier for modern historians to enjoy his work.
Tacitus was fortunate enough to have access to the Senate's official records, which gave him the inside scoop on what really happened there. His work focuses a great deal on the tension between the Senate and the emperors, so having the ability to access Senate documents was crucial to his understanding of that dynamic. However, there are portions of his Annals that are less trustworthy, as they come from mostly second-hand sources. The Histories were composed with greater reliance on the Senate documents. In addition, he also read speeches of emperors, and examined other literary and historic sources in composing his work. This large collection of sources that he chose to draw from makes the scope of his work as wide as the Roman Empire itself.
It is impossible for any historian to be truly objective when studying a period of history that is close to them, chronologically and emotionally speaking. For example, since Tacitus married the daughter of Agricola, it is likely that Tacitus's work on Agricola is very biased. Tacitus might have been unfairly harsh towards Domitian, because life under Domitian was a time of great chaos and intrigue for the Roman people. Overall, I would say that Tacitus is the greatest historian of Ancient Rome, and perhaps in all of antiquity. He manages to attempt, nearly successfully, to suppress his own biases in composing his major works, the Annals, and the Histories.