No one entered in 24 hours so I post the answers! Feel free to pick your cow the way home
Nr 6 Was Arthas with lvl 16 root and syndragosa upgrade. 1,25+ 1,25 +2s (but lets say 4s)
Maybe with mephistos root if everyone gets hit 1 by 1. Its technicly the longest. I didnt thought off that. Credit for meph goes to george.
Also jaina does 4.5s
That is incorrect. KTZ roots for 1,75 sec, and requires an enemy, that is under the effect of his ult to die in order to root again. In the best case scenario, you will be playing against TLV and Rexxar, so enemy team will have 9 bodies. If each dies as soon as the root expires, that is still 14 sec root (8x1,75, since the last one doesn’t root anyone after exploding). But such scenario is impossible to happen to say the least. Even if enemy team has these heroes, they will be dying by few at once, so the duration will be much shorter.