Season 19 is the worst for me, hands down, no contest. Worst winners, dismal cast (excluding Amani & Marcus, Andy & Tommy and some teams who were eliminated too early and maybe Bill & Cathi), pretty dreadful route (excluding Malawi) and I actually did not enjoy any episodes other than the two in Malawi.
Season 16 comes in at a close second for me, with another pair of terrible winners. Leg 4, 6 and 7 were among the best TAR has ever provided, but the rest of the season was beyond lacklustre.
Season 9 became boring with the 2-way domination. Season 10 had a terrible cast (except Dustin & Kandice and the early exits). Season 8 did very little right (except the first three legs, which I enjoyed). Those are the worst for me.
It's almost impossible to look at anything but Season 3 in terms of the Best Season. I loved everything about it. Seasons 5-7 were the TAR 'Golden Era', and they were perfect seasons, except for Freddy & Kendra's victory.
For me, the best post-All-Stars season is Season 12, with amazing legs on Leg 1, 3, 4, 5, 6, 7, 9 and 10, and even Leg 2, though I'd put it slightly behind these others.