Don't you mean Meganium is the one lacking special attack? You also forgot to mention the popular spore + focus punching Breloom ^_^. Now THAT thing is a physical machine, if only it could learn Earthquake...
I find poison and ice to be infrequent picks. Because just about all water types can learn ice beam and ice itself has several weaknesses, ice usually isn't needed. Weezing was popular for awhile, but with poison itself being only offensive to Grass (god, everybody picks on grass types), it's not that popular a type.
Bug still isn't that great of a type, only Heracross can get the awesome megahorn. But Volbeat can breed signal beam onto a few other bugs. Still, I think bug needs a moderately strong and accurate TM move. Being strong to psychic and dark makes bug an awesome type. Sure, there's a a few good bugs, like heracross, the ninjas, armaldo, and the steel types, but still too many which are almost unusable.
I agree with Frostweaver about Grass needing a poweful, accurate move (preferably a TM so others can inherit it) that is similar to ice beam, etc. I've been saying this ever since the GS days, mainly because I was horrified by razor leaf's much lowered critical hit ratio, so it went from being one of the best attacks ever to just another crappy one. Grass types are sadly treated only as powder puffs and usually have a difficult time putting thier high special attacks to good use. Not to mention water types can easily beat grass with an ice beam. Not many people appreciate Grass being strong to some of the most commonly used offensive moves, water, electric, and ground. I guess the fire and ice scares them off o.O.
As for rarest types, definately ghost and dragon. Dragon itself is difficult to aquire, and ghosts are very few and far between. Steel and dark are rare-ish too, but you can find steel and dark early in the game (like poochyena, aron, sableye and mawile). In GSC though, the average non-trading player will probably catch almost no steel, dragon, or dark types (since the only darkie you can get before you reach kanto is umbreon and it's tough, unless you play crystal which has sneasel), but you can find a gastly right in the beginning.
As for least popular by favoritism... well, I've seen that Ground, Poison, Bug, and Fighting are generally disliked. But I love Fighting, and I like Ground's electric immunity.