As others have said. The least used are Howling Banshees, arguably the most powerfull are Warp Spiders because of Infiltrator and First Strike. When using a 'fast attack' list, I nearly always use Shining Spears with the Avatar as it allows the commander ability to bring in other formations for a big assault at distance (up to 50 cms away). Spears really like to take on Leman Russ formations, and if combined with other bikes or Guardians in WS they can even ruin a titan's day

The other aspects can be mixed together to emphasise CC, FF, shooting, armour and mobility. The favourites are 4x Striking Scorpions with 4x Dire Avengers for a heavy assault that also provides good support capabilities. And you can use some Swooping Hawks or Shining spears to reduce the cost of mobilising the formation.
Following on from Steve's thoughts, a good shooting formation might be 2x Fire Dragons and 2x Dark Reapers with 4x Striking Scorpions.
My favourite is 2x Fire Dragons, 3x Striking Scorpions and 3x Dire Avengers for a heavy assault that also provides good support capabilities and some shooting.