A good Tyranid army reminds me of bears... If you see some little ones roaming around they might at first seem non threatening but you can be sure there's some big ones nearby... And they aren't nice at all. Following this train of thought there are already some early meta builds starting to emerge. Here is a typical example:
HQ
Tyrant - 2x Twin Linked Devourer, Wings
Tyrant - 2x Twin Linked Devourer, Wings
Elite
Zoanthrope
Zoanthrope
Venomthrope
Troops
30x Termagants
Tervigon
30x Termagants
Tervigon
Fast Attack
Hive Crone
Hive Crone
Heavy Support
Mawloc
Mawloc
If you're new to Tyranids this could be a good place to start:
•Flying circus (four FMC)
•Monster mash (eight monstrous creatures)
•Four troops with the ability to spawn more when necessary
•Zoies and Venom support and enhance the troops
The Hive Crone functions quite similar to the Heldrake, has some ranged shooting versus armor plus it can also drop into glide mode to assault weakened enemy units. Most important the Hive Crone is Ld10 so it can operate independently. While the Mawloc is rather hit or miss (no pun intended), it restricts your opponent's deployment which is strong. I think over time you'll see less Mawlocs as the meta adapts to counter it. This army list should be able to compete versus mid tier armies. While I appreciate the list I would never play it as it's too spammy for me, nor does it have that lethal melee edge I love.
Disruption
I believe that for Tyranids to be competitive versus the top meta lists such as Taudar they should play to their basic raw strengths that best counter these types of armies. Tyranids have access to a good number of broods that are both resilient and can arrive from reserve. If you have enough hardened disruption units arriving on turn 2 (hint - you should invest in a comms relay) you can overwhelm the enemy, forcing them to make tough decisions as to which broods they should first target, which prevents them from focusing on your troops.Versus Tau you must first survive the initial firepower from intercepting units (Broadsides and Riptides), one turn of enemy shooting (which is depleted due to their intercepting), then finally make it past their supporting fire during Overwatch when you go in for the kill to assault them. That is a total of three shooting phases. It might sound like suicide but I think it's possible based upon my most recent results.
Here are my current go to broods to disrupt the enemy forces:
Hive Tyrant with wings (deep strike-swoop mode)
Tyranid Prime attached to large genestealer brood (15+) with a Broodlord (outflank)
Large Ravener brood (6+) with the Red Terror (deep strike)
Tyrgon Prime (deep strike)
Spore Mines (deep strike)
You can also infiltrate genestealer broods, which is feasible if you've got some decent line of sight blocking terrain to hole them up behind for safe keeping until the reserved broods arrive. Be wary though of Tau smart missiles.
Spore Mines are a cheap distraction unit and they will cause some damage if left unchecked, thus forcing more tough decisions for your opponent as to what to shoot. The more pressure you apply the more likely your foe is to crack!
Four to five disrupting broods should be enough. Note that every brood listed above is Ld10 and there is some synapse too... They can operate independent of each other and don't have to worry about failing instinctive behavior tests. The key is to layer these units such that they support each other so that the enemy can't focus fire one brood with complete impunity to the rest. If they all come in relatively close together this prevents that from occurring and your opponent will definitely be hard pressed.
Here is a core list I've developed that is a disruption army:
Face Crusher
Hive Tyrant - Wings, 2x Twin Linked Devourers, Adrenal Glands
Tyranid Prime - Maw Claws of Thyrax, Flesh Hooks, Adrenal Glands
14x Genestealer & Broodlord - Toxin Sacs
14x Genestealer & Broodlord - Toxin Sacs
10x Devilgant
10x Devilgant
2x Zoanthrope
6x Ravener (rending claws) & Red Terror
Trygon Prime - Reaper of Obliterax, Regeneration
3x Biovore
Aegis Defense Line w. Comms Relay
You can start with up to six of these broods held in reserve and you're counting on at least four to five arriving on turn 2 with help from the all important comms relay. The big broods are all supported by tough multi wound creatures that can soak some wounds so you'll make it through to the assault phase next turn and take some names.
Conclusions
I've provided two army lists. The first is more conventional while the second is more aggressive. They are both sound but I feel the second list has better odds of taking down the likes of Eldar and Tau.