This has been debated many times over the years. Much depends on the intended strategy for the army and the potential targets. I disagree slightly with Steve on the better Aspects, but that goes to show that in general the Aspects abilities are well balanced.
The aspects that I consider better are Dire Avengers, Fire Dragons and Striking Scorpions. Dark Reapers, Swooping Hawks and Shining Spears are more specialised so less frequently used. Warp Spiders are generally considered the best because of Infiltrate, armour and First Strike FF attack, while Howling Banshees struggle because of weaker armour and the need to get into CC to get their First Strike attack.
- Warp Spiders are generally considered the best despite the transport nerf, as they have 4+ armour and First Strike FF. Infiltrate means that they can threaten a large area if placed centrally, but their various abilities mean that they do not combine easily with other Aspects.
- Striking Scorpions and Dire Avengers are the traditional Aspects for greatest impact in assaults. Their main drawback is the lack of shooting capacity.
- Fire Dragons are a favourite because of their MW assault and shooting capacity. They are also slightly contentious as the location of the Exarch and Autarch as there is a debate whether the extra attack also has MW. E-UK has allowed this for many years without issue, though I do not believe this has been adopted by Net-EA and I do not know the position of the French or other lists.
- Shining Spears main target is RA formations, though their mobility also makes them usefull objective grabbers. RA targets are usually limited which reduces their effectiveness a little, but they can be added to other Aspects reducing the transport requirements and 4+ armour makes them a better choice for Exarch and Autarch
- Dark Reapers provide ranged firepower against infantry, and this is also usefull for laying BMs. But their ineffectiveness against armour and single dice in assault does limit their use a little, which is why they are less frequently used.
- Swooping Hawks are usefull for teleporting into remote areas of the battlefield. Their single attack and weaker armour means that they have to pick on smaller formations and capture objectives, which in turn means they are more specialist, though they can also be used to reduce the cost of transport in mixed Aspect formations
- Howling Banshees are generally considered the weakest Aspect. This is because they are CC specialists with weaker armour. To reduce enemy impact on them, they need to destroy their target with First Strike, or pick on remote enemy formations which usually restricts their usefulness. This tactical imperative also means they do not combine well with any other aspect.
My favourite formation is possibly 3x Striking Scorpions, 3x Dire Avengers and 2x Fire Dragons upgraded to be Exarchs / Autarch and mounted in Wave Serpents. Note I use the E-UK lists allowing the formation 4x MW attacks in assault.
I also frequently use Shining spears and Warp Spiders with supporting Bike formations, but rarely use Swooping Hawks, Dark Reapers or Howling Banshees for the reasons stated above, though I have tried all these in the past.
