There are few factions in Warhammer 40,000 that embrace the sheer level of converting and kitbashing potential in the same way as the Orks.

My love of the Ork faction stems from the original Gorkamorka game in which the most stunningly diverse parts of the army came by way of the Trukks and Buggies ferrying warbands to the battlefield. With orks seemingly capable mechanical skills and their inability to comprehend the concept of "too much" you can create bizarre and unwieldy vehicles in which to rampage across the wastelands and battlefields

Ork Burna Bommer
Creator: @TheLilMek

Fuselage - Ad Mech Stratoraptor
Front wheels - scrapjet
Missile rack - Taruox Prime
Gatling Gun - Redemotor Dreadnought
Bombs and tail fin - Burna Bommer
Big shootas - Battlewagon Rear wheel - trukk

Deffrattla Buggy
Creator: KIT
Constructed from the Ork Wartrike with front wheels from a Rukkatrukk, powerklaw from the Ork Nobz Kit and a spare grot from the Mekgunz

Gargantuan Squiggoth
Creator: Mysteryblack2

Ork battlewagon, Baneblade parts, Imperial Knight legs, Leman russ parts, double diamon textured plasticard, jewelry chain

Megatrakk Scrapjet
Creator: Mysteryblack2

Stormtalon/Stormhawk kit, Ork Battlewagon, Basilisk bits, Drukhari razorwing, Meganobz, Cadian Heavy Weapons Teams, green stuff, textured plasticard

Shokkjump Dragsta (The Grot-Pod Racer)
Creator: Mysteryblack2

Stormtalon/Stormhawk kit, Imperial Knight, Ork flyer, Deffkilla Wartrike, Gretchin, cadian shock troops, Cadian Heavy Weapons Team, greenstuff

Looted Exocrine
Creator: KIT
Converted from a Tyranid Exocrine and parts from the Ork Deffdread Kit

Deff Traktor
Creator: KIT
Converted from the Ork Trukk with an Imperial Dozer Blade and Killakan weapon arm

Looted Tank
Creator: @Rictus40k

Converted using:
Astra Militarum
Chimera, Manticore rockets, IG wheels
Ork turret. Ork deffrolla bits
25mm base

Spida Killa
Creator: KIT
Gloomspite Gitz Arachnarok and Mekgunz Bits

Looted Raider
Creator: KIT
Drukhari Raider combined with Ork Killa Kans bits

Looted Walker
Creator: @Normnondo

An Onager Dunecrawler with a klaw from the trukk kit, KFF from the Morkanaut (no longer legal), driver I think from Morkanaut kit as well, and then just other random bits.

Creator: @AndyCRuns

Unique build using the Gorkanaut kit to give a Squiglike body construct.

Creator: @ZisKnow

Baneblade, Mek Workshop, Grots from a Scraplauncha

Looted Repulsor
Creator: @Rictus40k

Repulsor, Imperial Guard searchlight, Ork Skorcha, Zzap cannons, Battlewagon cannon, Burna Boy skorcha Stompa Supa-Gatler. Ping pong ball. Chimera hatch, GW terrain barrel, Baneblade cupola, Nob rokkits, Scale model pieces (cylinder pieces), Banner – Space Marine armour pieces, Ork Choppas, Kan ‘toes’, various pieces from bits boxes., Resin Tesla coils

Deffstrike Rokkit Launcha
Creator: @Rictus40k

Scale model rocket
FW Grot Bomb
1/35 scale tracks
Lego cone shape
Imperial Guard tank heavy flamer nozzles
Ork Big shoota
Various stowage from GW and scale model kits
Imperial vehicle command pieces
Additional Chimera hatches
Terminator teleport homer
Rhino (Mk. I) exhaust
Imperial Guard searchlight & grab handles
Rhino internal panel
Hunter Killer Missiles (original)

Looted Wagon
Creator: @Rictus40k
Leman Russ turret
Leman Russ sponsons
25mm Slotta base
GW terrain barrel
Ork Stikkbom
Deff Dread skorcha
Ork Big Shootas

Mekboy Junka
Creator: @Rictus40k

Manticore track units and lower & forward hull
Ork Death rolla bits
Battlewagon turret
Manticore rockets
Imperial roadwheels
FW Ork pintle weapons
Imp Guard searchlight
Sentinel searchlight
Chimera lasguns (just the ball joints)
Space Marine vehicle cupola
25mm slotta bases

Mekboy Junka
Creator: @Rictus40k

Leman Russ sponsons
25mm Slotta base
Baneblade exhausts
Baneblade cupola
Killa Kan exhausts
1/35 scale tank road wheel
Deff rolla parts
Ork big shootas (various kits)
Chimera hatches
Imperial Guard road wheel
Resin Tesla coils