I've always loved each instalment in the series for the unique things they brought to the table
GT1 was... well, the beginning, and is important for that reason alone, though I missed it when it was new
GT2 (my first GT) brought so many cars and tracks to use, great music, and plenty of races
GT3 brought smoother graphics and physics, and while it had less content, everything was meaningful and worked well
GT4 brought once again unbelievable graphics and physics for the PS2, boatloads of content and the slickest UI and music combination I've ever seen
GT5 was so incredibly broad in scope that it could never hope to present every aspect of itself with great depth, there were 1000 cars, though most were PSP imports, there were almost enough tracks, though many of our favourites had been removed, and while it couldn't reach the consistent quality that earlier GTs had, partially through unusual and seemingly pointless additions (XP, underused special events) it provided me and others with many memorable experiences.
here's hope that GT6 will improve on the lacking areas of GT5 and be a great return to form, best driving game ever? Sure, it's got the moon, why not?