Passive abilities have been OP in the last ver, however, in this ver as death strike is broken and all passive abilities have been nerfed, it might not be as OP as before, but passive abilities are still good abilities for both late game and early game.
Selecting the right hero for passive abilities is extremely important as you don't want stats gone into waste in int, this includes both stats gain and base stats at int. Heroes with good str and agi will make a very good passive heroes, heres what i think are the top 5 best passive heroes;
Hydralisk
Arthas
Illidan
Marine
Peon
You may wonder why i pick peon and marine as the best passive abilities hero, i will explain here why.
First of all, the first thing you want to look for in choosing a passive hero is the base stats in str and agi, add them up together, and you will find this 5 heroes have the highest number.
Hydralisk = 30str + 30 agi = 60
Arthas = 50 str + 8 agi = 58
Illidan = 31str + 27 agi = 58
Marine = 33str + 30 agi = 63
Peon = 25str + 29 agi = 54
Having the highest base stats in str and agi isn't enough, however, if we look at their stats gain we'll know further more who's the best in the long run.
Hydralisk = 3.0str + 4.5 agi = 7.5
Arthas = 5.5str + 1 agi = 6.5
Illidan = 4.5 str + 3.0 agi = 7.5
Marine = 3.5str + 3.0 agi = 6.5
Peon = 3.5str + 3.0 agi = 6.5
All the heroes above have the stats gain of at least 6.5 on str and agi stats gain, hydralisk and illidan has the best stats gain in terms of long run as they gain 7.5 per level, though assassin has the stats gain of 8.0 per level, it is far too weak for her base stats to catch up with others.
The third thing you want to look into is, their base attack speed. Base attack speed is extremely important in late game when everyone hits max attack speed, it is far more important than stats gain in late game. The calculation of base attack speed is mentioned in several sections, which is 5/base attack speed = number of time hit per second. A difference between 2.0 and 2.2 is 0.23, which is a 23% difference in damage assuming both heroes have the same damage.
According to glorn, most melee str has a base attack speed of 2.0, however, arthas has a lower base attack speed so i will assume arthas has the same base attack speed of hydralisk which is 2.25. Illidan and peon will have a BAS of 2.2, while marine will have a BAS of 2.4 or 2.5.
After reading this, perhaps you will look real close to these three importants factors when picking a hero with all passive abilities.