Ghost-type Pokemon, introduced as the Psychic-type killer, really suffered due to low distribution for in-game plays or suffering due to game mechanics not really favoring them (deja vu?). Due to a programming error (Gen 1 moment), instead of being a good offensive answer to Psychic-types, it turned out to be probably one of the worst answers especially since the only Ghost-type line was also Poison-type in Gen 1.
Speaking of which, the Gastly-line pretty much carried the Ghost-type even after this type was demoted from being rare due to the lines impressive offensive stat distribution (even as a first state Pokemon) as well has the defensive properties that got better with the introduction of the Levitate Ability.
Despite the lack of Ghost moves, not to mention them being physical, Gastly and its evolutions were still fantastic even in Gen 1 were Normal was most trainers only option of offense.
When Dark-types were introduced, offensively both types were almost redundant with Dark being a special offensive counterpart to Ghost for a while but defensively, until Fairy-types came, they removed each others weakness (thus Sableye is impossible to defeat and is obviously in everyone's Gen 3 in-game team imma right?).
In my playthroughts through many Pokemon games, I have never seen a type that was completely dominated by 1 evolutionary line for the most part. Even if you could not evolve into Gengar, in a game where you could get both Gastly and any other Ghost-type, I still think Gastly is the superior pick. But whenever I do pick a Ghost-type, I still don’t pick Gastly when I am not able to evolve due to my desire to have a fully evolved team in my playthrough.
In Gen 1 and 2 there were no other options (Misdreavus is a Gen 4 Pokemon and you cannot convince me otherwise), Gen 3 the Ghosts were late game, suffered greatly from stat distribution or being late game (Sableye is good for Brawly tho and unbeatable), Gen 4 even Rotom seems kinda inferior to Haunter and Mismagius probably being the closest thing to keeping up with it and I would say even Giratina is worse than Haunter (Giratina has higher staying power but if you used Giratina in Platinum you know its not only agony. Giratina needs lots of investment to benefit of its defensive stats and haven't find a way to get the Gracious Orb before post-game thus never used Origin Form in-game). In Colosseum Misdreavus benefitted due to circumstances similar to Gen 1 Gastly (and Spell Tag physical Shadow Ball obviously turns the playthrough trivial).
Gen 5 I don't remember much about Ghost-types besides them being pretty late and for the amount of investment not very worth it.
This leaves Gen 6 onwards where I think the first really good competition is Aegislash giving you a lot of flexibility on your playstyle or how you want to play through the game. Like Dark moves, Ghost move became more potent too, however, like Dark types where aren't many good options with base powers over 80. Physical Ghost moves are especially hard to get by with the best hard hitting option being Shadow Claw (70 base power). So even the 'blessed' Mega Banette would still not win a spot in my in-game team.
In a general sense, I think despite on paper Ghost-types getting better (mostly offensively), defensively speaking they might have gotten worse and I think in the earlier paragraphs I kinda hinted on it.
While Gen 1 you are weak to the broken Psychic-type, you are immun to Normal and the fact very few Pokemon even carried STAB moves until even Gen 3, gave the type long staying power, like Colosseum Misdreavus. As such even in Gen 2 where you might think Ghost-type is at its worst, because getting good special moves on Haunter is a nightmare, you still get a lot of utility like Meanlook + Hypnosis against the legendary beasts or generally wearing down problematic Pokemon. More such advantages and probably why the Gastly-line is taking advantage of every strength of the Ghost-typing in every generation (I have played so far) I listed in this post
I think especially for in-game the Gastly-line literally carries the whole Ghost-Type and for good reason. Every generation they abuse another mechanic.
While in Gen 1 there were no good Ghost-type moves (not to mention they were physical and didn't work on Psychic-types anyways), they high special stat, good speed even as Gastly as well as being immun to one of the best types in...
What are your thoughts about the Ghost-type? Am I overhyping it or give too much credit on Gastly?

