Two units that are among the fastest units in the book with good to reasonable shooting and solid melee combat lets look at them!.
Hellions are a high strength unit that excel in any phase of the game, they are in hard hitting in melee, have 2 poison shots each and have access to a strat to kill people in the move phase. While they are expensive at 17pts a piece they are movement 14", have a free floating stat point from combat drugs, come with a damage 2 melee weapon and have access to the wych cult obsessions in the right detachment. There's really allot to like about these guys the capacity to be toughness 5 really adds allot to their survival, it's one of those break points in game where small arms cease to be as effective. Simultaneously the ability to be strength 5 in cursed blade also pushes them over the top in melee. You have other options for them of course, 4 ways to improve their offence with +1 to hit in melee/shooting(though I would skip the melee), a plain old attack(a great option), strength (soso but with hyper stimm you can hit strength 7 which is hilarious and actually useful vs the odd predator, also good for no cursed blade).
As a unit on the battle field due to the hit and run rule they are impossible to pin down, you cannot tarpit hellions they ride free and unfettered over the battlefield going where they please!. I will be flayed alive if I don't mention Eviscerating fly by here, it's phenomenal for Hellions they make easily the most efficient use of one of our most powerful strats. The helliarch also has access to a unique weapon in the stun claw but it's only ok, the basic hellglaive is good enough.
In regard to obsessions they benefit from everything pretty evenly, nothing tricky here hellions are a hammer and will be good no mater what you give them (test of skill will make you a big boy buster even if you are only strength 4). Did I mention that skyboards are also cool as all hell?.
The fastest none aircraft in the book, another unit that can be strength/toughness 5, with multiple attacks. They have 2 options for special weapons the blaster and the heat lance, the heat lance is easily the better of the two, while it's true you can advance and shoot the blaster under most circumstances that shouldn't be necessary. This is where things become a bit more interesting in the combat drugs department, 2-3 heat lances that hit on 2+ is attractive for tank busting and combined with some of the other gear reavers become a unit that kill tanks really easily. Reavers have 2 options for handing out mortal wounds only one of which is useful, the grav talon is d3 mortal wounds on a 4+ if you have 2-3 of those the damage is good for finishing units and can outright kill light characters.
Obsession wise there is the standard stuff strike first, +1 strength, re-roll charges its all good but there is one obsession that I have a soft spot for. Agile hunters with hyper stim with give you a move of 24" the ability to get across the board like a ballistic missile and deliver a hard hitting unit like reavers into their heart turn one, suddenly that expensive unit that thought it was safe is now dust from 3 heat lance, some grav talons and a bag of blade vane attacks(and if the target flys you get +1 to hit). You can alternatively leave them in cover out of sight and then pick up your +8 advance move for a total of a 32" move, excellent for grabbing support units.
All in all fast and hard hitting both are units I adore, potentially more resilient than most of our other units, with careful application they can be an absolute wrecking ball on the battlefield.
Next time Beasts!