Attack bikes. No hesitation, not even a sidelong glance if there's only the one slot open....
Yesterday, my 3 attack bikes (in two units) were the men of the match. They were able to outmanoeuvre roughriders and reduce the squad to only two guys. They then weathered the charge, and went on to stun, then kill a basilisk (with Heavy Bolters!) After this, they mopped up the only surviving IG vehicle left by rapidfiring into side armour, stunning it and startling it to move thru terrain. This promptly immoblized him and the game ended with my Attack bike beating on his side armour, claiming his table quarter in Gamma Cleanse...
A great unit for three reasons;
-they're fast and can still pump out lots of shots (4-5 shots, two rerollable at short range) They can dictate whether they want to shoot up a close assault unit from afar, or shoot and charge something shooty. This is something bikes can't really do.
-they have the speed and survivability to use there anti-infantry firepower against armour (guess what...a Leman Russ is still AV 10 is you shoot him in the @ss!)
-they can turbo boost and get an 3+invulnerable save with toughness 4 (5) (great when the dev squad they're advancing upon forgets this instead of firing on a more worthy target.
-they can also tie up said devastors/ heavy weapons squads in assault. They have two wounds and two attacks base. They could turbo boost TWO FEET, hide, next turn move 12",rapid-fire AND then still charge!
On top of all this,they are not even close to being overcosted.... What more could a guy or gal ask for?
Not that bike squads are horrible, indeed you can come up with some nice niche units. I wouldn't mind taking them for a fun, themed game or if I am bored on vassal and want to try some stuff out. They have all the qualities I have listed except they lack anti-horde shots (at a distance that keeps them safe from assault) and are a much more tempting, rather expensive target. They are also harder to hide behind terrain as a unit. Alas, when it comes time for making a proper list,there just isn't any debate in my book ...