I almost always play engineer so my kit very rarely changes. Most of the time i am driving a tank so i planned my kit for if i were to get separated from my vehicle and crew. What kal said about the ECH and his FAKs is 100% true. I don't play with stamina so i generally run a very heavy kit (This one comes in at about 85% of the weight limit)
Uniform
CTRG uniform, CTRG heavy plate rig, Assault pack, ECH
I use the CTRG uniform because i prefer it's look over the others, also the heavy plate rig has high armour values
Primary
MX SW; Bipod, Flashlight, ARCO, 100rnd red tracer mag
As i said before, getting separated from my tank is what i plan on, that means having to fight off enemies untill i can get to allies. The MX SW is perfect for that, it gives a single soldier a lot of fire power and can make defending a HQ a hell of a lot easier. I use red tracers since it makes fighting targets at long range easier (Burst fire and watching where the tracers land)
Sidearm
4-five. 45 ACP; MRD (Red dot sight), 11rnd mag
I think in the entire time i've been playing ArmA i've only used my sidearm once and that was back in A2, however i will never leave without one, it could save your life one day.
Gear
NVGs, Rangefinder, Tactical shades, GPS (Very important)
2x RGO, 4x 11rnd pistol mags, 3x 100rnd red tracer mags, 3 FAKs, 3x white smoke, 2x green smoke
^^^This equipment is standard on any kit i play
2x SLAM mines, 2x Explosive Charges, 1x Toolkit
^^^This is my engineer specific equipment
NVG's, rangefinder and a GPS are a must for me, So many times i've had people asking for NVGs when night pops up on the server. A GPS is extremely useful when driving a vehicle as it allows you to not have to pull the map up every 100 meters. I have 3 spare MG mags giving me 400 rounds to defend myself with. Always carry atleast two smoke nades, they can be invaluable in close quarters and in open areas when you need a little bit more cover. Having coloured smoke on hand just adds a bit more versatility.
Onto my engineer gear, the toolkit is obviously a must, this cuts my backpack space in half, either i take one satchel or two explosive charges, i picked the charges as they are slightly more versatile(eg. Clearing obstacles in a town) and one satchel cant take out a radio tower. To supplement this i take two SLAM mines in my vest, they can immobilize most vehicles (Although they wont kill them)