(Passive): +3 ability power per level, +220 health and +6 health regen per 5 seconds. Power Transfer increases Viktor's movement speed by 30% for 3 seconds. OR
(Passive): +3 ability power per level, +200 mana, +10% cooldown reduction, +5 mana regen per 5 seconds. Gravity Field has an additional 30% cast range. OR
(Passive): +3 ability power per level, +45 ability power. Death Ray sets fire to enemies, dealing 30% additional magic damage over 4 seconds.
That's what? 54 AP passive? That's pretty good actually. But then again, it does take up one of his item slots... Oh well, very few games actually fully complete all item slots anyway. I'm guessing each passive doesn't actually make it +6 AP per level or that'd be pretty freaking broken.
The first one seems pretty nice if you're going tanky build. The stats are honestly kinda meh and if it were an item, I'd say it'd only be useful in the early game. The movespeed is nice though and I know his Q is really spammable... so it's essentially a permanent movespeed boost.
Second passive seems really shitty honestly. Maybe if he was support? But even then... I dunno... I don't actually know the cast range of his W, so maybe it's essential.
The last one seems the best by far. That essentially gives him an item with 99 AP. I much prefer that to extra mana (though the HP one is good too). Boosts his nuke quite a bit too.