1981 - Can't argue too much about the legality of the Brabham. But the 1983 championship is clean - there was absolutely nothing illegal with the fuel being used by Brabham (which has been explained at length in another thread).
What "preferential" treatment did Piquet have in 86? Mansell had Patrick Head as his race engineer and there were no team orders, despite Piquet having signed a contract to have No 1 status (which should have meant preferential treatment).
Mansell should have won the 87 championship at a canter after Piquet's massive shunt at Imola. The fact that Piquet could still win the championship with races to spare despite suffering from constant severe headaches, insomnia and a lack of depth perception is testament to his ability.
If you want to indulge in how many races each driver could have won, then Piquet could have won 2 more races in 86 (Mansell got lucky at Brands and Prost got lucky at Adelaide), whilst Piquet could (and should) have won at Silverstone in 87. But for Jones barging into him in Canada 80 & Prost's fuel gamble at Adelaide 86, he could have been a 5 time champion.
It is interesting to look at people's perceptions of Mansell & Piquet. Mansell is always judged mainly by his 1986 - 1992 form, whilst his 1980 - 1985 form is (comparatively) ignored. Piquet is judged more by his 87 - 91 form whilst his 80 - 86 form is ignored. Thus, Mansell is usually judged to be a "better" driver. Add in the ignorance and jingoism of the Great British Press and bingo - Mansell's a legend and Piquet's a crybaby.
Simple fact is that Nelson was the class of the field from 81 to 85, and was still at the top (along with Mansell, Prost and Senna) in 86 & 87.