30K Mechanicum

[The Supreme Emissary]

Mechanicum Taghmata
Perhaps some of the most desirable models ever made. The ancient Adeptus Mechanicus features living war machines of terrifying power and resilience. This is your base list that brings your whole catalogue together. 
Always led by a Magos of some variation a Mechanicum army is highly customizable and pretty much allows you to play them exactly how you want. However, as a still expanding faction in game terms, their full potential and regular play has yet to be defined.

Tech-Priests
As low as an Auxilia and as high as an Arch-magos these hooded cyborgs come with a ton of options to fit the job.
Servitors
From the claw and cannon style slave bots you've seen for a long time to the blobs of Adsecularis combat thralls, servitors are the gribbly you hate but want to love. Thallax and Ursarax fall in this category but are a breed apart, serving as highly mobile heavy combat cyborgs.
Automata
This is what it's about, Robots that are actually Monstrous Creatures that share functionality with vehicles. There's a selection of bots that are still expanding, but they're already gorgeous models waiting for any excuse to smash the battlefield. 
Vehicles
A selection of repurposed/modified imperium vehicles and some unique pain-tractor style battle tanks and transports. But they're best in show is the massive and monstrously destructive macro-engines.  

[The Supreme Emissary]

Legio Cybernetica
A Mechanicum force led by a Dominus made up primarily of Automata. The advantage here is bringing the upgraded Dominus who can join automata units and doubling the range of command. Cyberthurgy can make your bots do some pretty cool tricks and you are the best at using them. You can still bring other kinds of units but you need to have an automata in that slot class first. 
Paragon of Metal is a great upgrade for solo bots so you don't have to keep a leash on them but this force is built on groups of bots, so save your points. You're playing a nidzilla style army that know show to shoot.

HQ - Archmagos Dominus
He can join a unit of bots and keep them repaired
Troop - Castellax
You have to take these in pairs so maybe plan on having 4 to work with. This also lets you work with them in a marine detachment however you like. 
Elite - Domitar
Bigger and meaner with a focus on crushing buildings.
Fast Attack - Vultarax, Vorax
Vorax are your cheap grubbily style option. Vulturax are your only current flyer option. 
Heavy Support - Thanatar + +
Thanatar pack heavy fire support weapons and can defend themselves nicely. What they lack is volume of fire which is where your cyberthurgy rolls can come in.

[The Supreme Emissary]

Ordo Reductor
This army lives for 3 words, "Big, Badda, Boom"! They live to smash fortresses into dust, scatter entrenched units and turn enemy vehicles into liquid slag. They can march through ruins with easy and blast most things that move to vapor. Thallax are your go to infantry and are Jet Pack infantry with heavy infantry profiles. What you won't have is a lot of focused melee ability and your defense will be far less impressive than your offense. 

HQ, HQ
Troop, Troop, Troop, Troop, Troop
Elite, Elite
Fast Attack, Fast Attack
Heavy Support, Heavy Support, Heavy Support, Heavy Support
Lord of War, Lord of War

You have to take a Arch/Magos Reductor as your compulsory HQ. 
Your army can't be pinned by the enemy and ignore the effects of moving and charging through difficult/dangerous terrain. Compulsory Troops must be Thallax. No more than half your detachment can be made up of battle/siege automata units. 
Your detachment can feature all the Taghmata units with a few additions and some restrictions. 

Magos Reductor 125p+
Has Sunder and adds +1 to the Vehicle/Building damage table. He can give up his shooting to give this ability to a Heavy/Ordnance weapon fired by a friendly unit in 6". Can be accompanied by a unit of Scyllax or Tech-Priest Auxilia (Reductor) as a retinue unit.
Ordo Reductor Artillery Tank Battery 85p+
Build your own cannon battery! 1-3 cannons of all kinds for marching fire support or back line bombardment.
Ordo Reductor Minotaur Battery 205p+
Formerly an imperium LoW choice, here this rare unit can be taken in groups of 3 and given a selection of defensive gear. 

[The Supreme Emissary]

Secutarii
These are the Titan Guard. Skitarii models upgraded with even fancier gear to take the field next to super heavy walkers and keep sneaky buggers at bay. They can be part of a Taghmata or Ordo Reductor list but they really want to be there with a proper Titan in tow. 

Secutarii Axiarch - Your new HQ option that can starts to look like an Alpha but turns into an Astartes Captain that can buff his underlings.
Peltasts - Gunner style guys that you can make shift with Skitarii Vanguards. 
Hoplites - Arc spears and shields to fill melee duty.