I typically only take a Tervigon as an HQ in medium points games. Between 1250-1500. I almost always play at 1850, so I almost always field them as a troops choice.
My HQ choice depends on the list I am running, but usually ends up being a Hive Tyrant. In my triple Trygon+double troop Tervi list, I run a vanilla Tyrant with HC for the reserve bonus, and I will occasionally use it to outflank one of my two Gant squads (the other gets put in normal reserve for late game objective capturing). It happens to be the cheapest way to get the reserve bonus for turn 2, and the Tyrant can still fight decently enough as is. In my Flying Wing list, I go big with him: my Super Old Adversary Dakka-Flyrant (AG+2xTLDev's). He typically stays at the back of my cover phalanx, but springs forward as needed to support my Shrikes/Raveners. Jumping over vehicles and blasting their rear armor is always fun, and with OA and AG, he is a fearsome foe in combat. One giant glass cannon to add to the collection featured in that list.
I have actually fielded the Parasite on several occasions. He can be extremely fun to play with, particularly against GEQs. There is something just priceless about a lone Parasite in combat with an enemy squad, and then BAM, they are completely surrounded by Rippers. How much good those Rippers actually do is debatable, but it certainly gives a nice psychological effect. He is more expensive than a Prime but cheaper than a properly equipped Tervigon/Tyrant, and he usually prefers a squad of Shrikes or Gargoyles to accompany, but if your Troop Tervies happen to spawn a good squad of Gants, he can stick with them for a little while and jump out at an enemy squad when they get close.