Now that most people have gotten their hands on the new Tyranid Codex, you might be wondering about the best way to new or current gribblies. Warning, some mathhammer might be involved to calculate the most efficient option. We will also only be going through the more popular units that have a bunch of weapon options available to them.
*A Neurothrope directs the swarm
All the...Small Things (Warriors, Gaunts, Raveners, Hive Guard)
Warriors - Deathspitters and Boneswords are your best combination here. At a cost of just 1 point more than regular Devourers, Deathspitters have +1 Str, -1 AP and a 6" longer range. A better option hands down on anything that has the option of both shooting weapons. The extra attack from Boneswords also make them the better choice over Rending Claws or Bonesword lashwhip against even 2+ save units where one might expect rending claws to be better against. As for the bio-cannon that you can bring for every 3 warriors, the much-improved Venom Cannon is a better choice simply because anti-tank weaponry is harder to come by for a Tyranid force.
Gaunts - 2 main ways to outfit them. On the screening, and thus expendable, version of Termagants, Fleshborers are arguably better because it keeps their cost low and performs better with access to the Scorch Bugs stratagem compared to Spinefists (which admittedly, can fire while the bugs are in combat). For the anti-infantry role, nothing beats the whopping 90 S4 shots that a full unit of 30 Devourer-armed Termagants can pour out. These cost double the points of a regular gaunt though so you need to find some way to deliver the payload.
Raveners - Depending on their role, double scything talons vs GEQs and Rending Claws vs anything with a 3+ save or better. For ranged weapons, as mentioned in the Warrior entry above, nothing beats Deathspitters on units that can take them.
Hive Guard - Against vehicles, the shock cannon still comes out ahead in terms of efficiency because of its special rule of causing mortal wounds on a wound roll of 4+ and d3 mortal wounds on a 6+. However, I have to go with the Impaler cannon for all round effectiveness and utility as it removes the need for line-of-sight and has a range of 36" compared to just 24" for shock cannons. They also do much better against non-vehicular targets.
*Poor Timmy broke his nail on his first day out
The Big Things (Tyrants, Flying Tyrants, Carnifexes, Tyrannofexes, Harpies)
In contrast to the small bugs, you'll generally want Devourers with brainleech worms over Deathspitters with slimer maggots in the anti-infantry role for any model that has the option of taking both. The former might have 1 less point of Strength and AP and 6" less reach but more than makes up for it with double the number of shots at the same cost.
The Stranglethorn cannon has an interesting spot in all of the options as it does 2 damage. That makes it unusually good at killing Primaris Marines (especially if they are in squads of 10) and Assault terminators with storm shields. The Devourer with brainleech worms is better at killing the smaller stuff and Heavy Venom cannon, larger targets.
There's only really 1 weapon option for the anti-tank role for your big bugs that have various options and that's the Heavy venom cannon and its buffed damage of a flat 3 now. Ironically, that also makes it ideal for dealing with other medium-sized Tyranid threats (warriors, raveners, etc) and Obliterators which come with 3 wounds each.
Tyrannofex - Running the numbers, the Acid Spray is surprisingly better against all targets than the other 2 options EXCEPT against T8 targets where the Rupture cannon pulls ahead. Do take note that the Rupture cannon does have more than double the range of the Acid Spray which allows it to start firing earlier (and start double-firing earlier as well) unless you drop the Tyrannofex down in a Tyrannocyte. The acid spray does have the bonus of not having a degraded hitting profile (its strength does go down) once your T-fex starts losing wounds.
For melee options, Monstrous rending claws come out tops against most targets and has the benefit of not costing any points. The consistency of a flat 3 damage from Monstrous Scything Talons compared to d3 for non-rending wounds from the MRC however, fares better against targets with 3 wounds or more. While Monstrous Boneswords do give an additional attack, losing re-rolls to wound and a point of AP makes them worse than MRC against all target except for those with only a 5+ armour save or with an invulnerable save less than 3 steps away from their regular save (ie: Celestine who has a 4++ and will only be affected by at most, 2 points of AP). In general, the MRC comes out as a better all-round melee option for both Tyrants and Carnifexes.
I hope this light analysis helps in your decision-making with regards to outfitting your swarm for the new codex. Many of the options do have their uses so do take the role of your unit into account before making the final decision though the above choices are solid takes in most cases. If you've not read The Crossroad's First Impressions article on the new Tyranid Codex and what our top picks are yet, you can find it here.