Assault: My favourite class ever. 1-2 become marauders, rest are offensive psi. Sometimes for funsies I gene mod someone. Never officers, because they are too brash and I RP my officers. Power spike at SGT (super early!).
CCS at LCPL is exceptional, but can actually be a slight liability with the 3 ammo laser shotgun. Once you get gauss / ammo cons it's okay, but still can be hard to manage your shots early on. Still, it's the best choice. Next is aggression because I'm an angry, angry person, and it gives me like 96% crit with flanking and a shotgun. I'm pretty sure WTS is a better choice, but I give my assaults lots of heavy armour to help compensate for my critlust.
SGT is close encounters. It is about 1000 times better than the other options. Every time I've picked another I've regretted it and chopped that assault. Bonus will is gravy. Resilience next, again to help compensate for my overly aggressive playstyle.
Bring em on is an exceptionally, exceptionally strong choice for an assault and everything about this talent is awesome. They're up front and can see most enemies because of it, they frequently have very high crit chances (see above), and they love murdering aliens. Unfortunately, sprinter is better so I take it. But I wish I could get BeO too. MSGT is EC, because more health and more aim and more will and more speed. LR is a very strong choice, especially if you're not picking it on your pathfinders and you hate scouts. Unfortunately I love my scouts so I never pick LR.
Equipment is shotgun, sometimes an assault rifle, the heaviest armour I can find. Early I'll take alloy plating and flashbangs, later I'll take running legs and shredder ammo.
Engineer: These guys are super useful and flexible and great additions to any team always. I wish I could find a good shogun build because early and late engy builds often need different things. That said, as it stands my engies all become defensive psimons. Power spike at LCPL because sapper.
LCPL is sapper, always. I used to take smoke grenade sometimes, but don't anymore because my medics all have it and I miss not being able to blow up cover. Similarly, next rank is smoke and mirrors. HEAT grenades are 'meh' and you're almost always better off cheming or taking out cover. I always take repair because I love the rush of capping aliens, but suppression is a REALLY strong choice on these guys and I wish I could bring myself to forgo the capture high a few times and take it.
Battle scanners, again, are the best choice almost always when available. EC and packmaster are both exceptional. HEAT ammo is terrible since I often use SMGs on my engineers and also they have like 7% aim all game. EC gives you MUCH more frontline presence and lets you be a formiddable shotgun carrier. Packmaster gives you unlimited grenades. MSGT is bombard, nothing else even comes close. Engies aren't damage dealers and their aim is so awful that rockets would be a liability.
Equipment can be almost anything. Assault rifle for mid range stuff, SMG for super mobility and grenade chucking, shotgun for up-close work that doesn't require good aim. Armour is usually heavy cause these guys often have to get up close to chem shit or shoot bad guys. Items are almost anything that can be thrown, with some occasional alloy platings thrown in (haha, thrown in).
Medic: Last but not least. These guys all get psi, no matter what. They can't shoot for shit so I don't bother. I love me some health packs, though. I don't bother with guardians. Power spike is mind merge.
LCPL is a pretty even split between suppression and steadfast, actually. The former gives me more versatility, the latter gives me more aptitude. Breadth vs depth. Do I want to option to suppress when I don't want to psi, or slightly better psi when I do psi? Since I mind merge almost every turn when it's available, I often end up choosing steadfast. That said, when I pick suppression I never regret it, so go nuts.
Smoke and mirrors, smoke grenade, and dense smoke as the next three, with the sole exception of sometimes one or two combat drug users, just because I like the glitter bombs. Paramedic and revive are exceptional, but glitter bombs and +40 defense are better.
I used to always take sprinter, but one day I realised that I wasn't often moving much with medics, just basically staying safe at the back and mind merging, psi'ing, grenading, etc. So I pick bombard now and I LOVE it. It's great. Saviour and packmaster are both exceptional and highly situational. There are definitely times where either would be a waste. Ec is sexy too. Similar to infantry, I don't feel that there's a wrong choice, here, but I usually go for packmaster.
Equipment is usually mid-light armour, and whatever weapon is left over. Medics can use shotguns, and do sometimes, when I'm feeling crazy. Typically they get SMGs, carbines, or assault rifles (the latter especially when they have suppression). Items are support grenades and med kits. Rarely do I take alloy plating, since my medics are low priority targets. Suppression medics get them more often, though.
Well, that was stupid hella long, but there you go.