Welcome to Librarium Online!
I'm planing to play a Apocoliptic game and want to know what you guyes think of the Diffrent titans.
Yeah, I feel like the "bio-titan" youre referring to is probably the hierophant. I don't believe hierodule or trygon are true bio-titans, they're just huge MC's really.
However, you are forgetting the Harridan, the huge winged bio-construct (I think it is "titanic" or at least a super-heavy.
I voted hierophant- that thing is a real showpiece and a nightmare to face.
I like the trygon. While it isn't huge, I like the idea of it popping up in the middle of the enemy and reeking havoc.
I vote Harridan.
Nids & Guard
GMail = MVBrandt
I think it really depends on what you want, do you want the nastiest thing around? Or do you want it to be easy to use or simply effective. The Hierophant is by far the nastiest of the lot. However the Hierophant is also the most expensive as far as points go. This means it will eat into your points allowance much more. I am working on putting a Hierophant together, off and on, at the moment. It is a very nice model with lots of details and will look really good when finished. However it isn't in any way easy to to put together.
I think that the two Heirodules are good in that they aren't as expensive aa the Hierophant. The Barbed Hierodule is good both at shooting and assault and I think will suit platers who want to be flexible. The Scythed Hierodule is as far as I know a giant assault beasty. If you like that I suggest go for it. The Hierodule models are by far the easiest to put together. However they aren't as well made as the Hierophant and Trygon, basically they have less details and the Tyranid look wasn't as refined when they were sculpted as it is now, the Harridan is debatable.
I don't know how the Harridan behaves in Apocalypse. However I remember that point of point it was the worst Tyranid, possibly one of the least useful of models in the whole game. This was due to it being a horribly high points value but only having the ability to fly about. It's guns were no better than a Barbed Hierodule. However unlike Hierodule the Harridan wouldn't present a constant threat as it flew off the table at the end of the turn. The model looks ok but that's all, the Hierophant is cheaper and far superior.
I think my favourite is the Trygon. The Trygon is handy because it is cheap compared to the others. I haven't seen it's new rules yet. However the Trygon was clearly one of the best because it is simply easy to use. Fast, and not too expensive but still powerful enough to butcher many people. The model is also one of the best. It has loads of detail and looks very good. I really engoyed putting mine together and painting it.
I think that it you aren't sure get a Trygon. It costs less money than the others and it is easy to use. If you want a larger one get the Barbed Hierodule. They aren't really fragile though some have said they look fragile. It also provides a good all-rounder. There is also the Scythed Hierodule if you want assault punch. If you want huge and horrible then get a Hierophant. The only one I would say you avoid is the Harridan. The model is only ok and I don't know about the new rules but the older ones were terrible.
I think the Harradin model is sick!
It's a big flyey killy thing!
But I vote Hierophant, if not just for shock value.
In apocalypse, flyers are hard to hit, but do not leave the table. The Harridan has 2 pairs of 6 shot s10 ap3 cannons ... yeah. The one thing about gargantuans is that in apoc games there's so much anti-tank firepower on the table it's pretty easy to focus fire and gank a model w/ a removable saving throw.
Nids & Guard
GMail = MVBrandt
It's cool, it's creepy, and can digg to anywhere in the map without dying for a stupid scatter roll 8Y
And in my oppinion, the model is the coolest of the three
Tyranids Win / Draw /Lost 
Daemon Hunters Win / Draw / Lost 
My bad on the whole "Titan" thing. IMO anything bigger then a Landraider and walks around is a titan to me. But that might be fom the days in highschool.