As the longest serving Doctor (to date), Tom Baker absolutely dominated this role. For many, he IS the Doctor. A genius like all of the previous incarnations, this one was also eccentric in the extreme and it was inevitable that he would steal just about every scene he was in.
He was the second Doctor to "break the fourth wall" although it was done so subtly, most were unaware of it. (clip recently reshown as part of the Confidential series, where Tom ad-libs that not even the sonic screwdriver would get him out of this.)
A great deal of Time Lord 'lore' or canon was explained during his tenure - for instance, Omega and Rassilon, the eye of harmony, the Matrix, The Guardians and the limit on the number of regenerations. Some would say that parts of this were later contradicted or became unwieldly, but whichever way you look at the programme, Tom Baker is still the first person to spring to mind when the subject of DW is mentioned.
Previous work included a major part in The Golden Voyage of Sinbad (playing a baddie). When he left the role, he went on to make a fortune from voiceover work, and of course he's known as the narrator in Little Britain.