The issues I have with Gengar being in the top 5 (let alone top 3) is that it took until DP for Shadow Ball to work alongside its SAtk stat. It was the STAB move and Focus Blast which helped Gengar in 4th Gen to excel, but was never going to really take full advantage of its new found power because Choice Scarf, Azelfs and the like were on the rise; not to mention Scizor's Bullet Punch boost thanks to Technician during the Platinum era did not do it any favors. In RBY it was outclassed by Psychic-types and weak to common moves such as Earthquake (the only favorable Normal-type immunity was always accounted for, as there was no such thing as Choice locking yourself to the point where you can take a turn to get a foothold in the match). GSC might've been where Gengar was considered one of the most useful Pokemon as there were still a shortage of Ghost-types and it was the only meaningful one of which could block entry hazards and maintain an offensive presense with gimmicky Perish Trap sets. In ADV, Gengar was all the while very versatile when it came to inflicting status and abusing BoltBeam-like coverage, yet was not most reliable spin blocker thanks to Dusclops' bulk proving to be more useful in stall, and the likes of Starmie could outspeed Gengar and still Rapid Spin hazards successfully; this was also a common scenario during the DPP era. I'd give Suicune and Starmie the nod over Gengar, with whom and Zapdos fighting for 5th place.