IIRC the reason Counter was used on Gengar in GSC was to bounce back Earthquakes from things like Snorlax that fell just short of OHKOing, but now Gengar either gets hit by special attacks or physical attacks that OHKO.
how's the EV spread of this semi-defensive gengar?
Nature: Modest
184HP/128Def/196Spd
Stats at lvl 100: 306 HP/188 Def/331 SpAtk/186 SpDef/305 Spd
Moveset that I have in mind
@life orb/expert belt/wise glasses
counter/focus blast/hypnosis/destiny bond
thunderbolt
shadow ball
HP ice
Even if gengar has no EV in special attack, 331 is still a big number. With one of the items I listed above, the slight decrease in special attack is not going to affect gengar's special sweeping ability much. The rest of the EV is spread between HP and Def. It gives a good surprise to your opponent who thinks they can OHKO gengar with their physical attacks. Of course,this gengar is not meant to take hits from guys like garchomp. (I think it can take a stone edge from a jolly garchomp though, if I used the damage calculator correctly.)