What are the most effective attack compositions to focus on general?

Because research costs of troops are really high and takes a long time as levels increase, it is more valuable to focus on a particular set of troops for both war, farming and castle defense.

• Farming: So it is generally accepted that barbarian, archer, goblin, wall breaker, minion, giant + healing, rage are good for farming.

• Defense: Archer, wizard, dragon, minion are good for castle defense.

• War: All dark elixir troops, pekka, dragon, healer, balloon, wizard, wall breaker, giant, archer + all spells are good for war.

While we don't have too many choices for defense, we have a lot of tactics that can be used on offense with mixing different troop and spell combos. Various types of barch, tank + dps, tank + healer, all air etc..

I am looking to only upgrade and only train a minimum amount of troops but still be very effective in all aspects & stages of the game. Theorycraft based answers would be appreciated or Town Hall9+ players with experience.

So what is the best attack compositions to focus on general?

The game's meta shifts pretty quickly, so unfortunately there's not just a few types of troops that you can upgrade and hope to be ready for every situation. My advice would be to practice some of the major strategies. When you come across one that is particularly effective and you can deploy it consistently, make sure you focus on unlocking the upgrades for that particular strategy first.

Let's be honest as well - for most people, the only thing that consistently pays out is collector farming with cheap armies. Clan wars and trashing bases for storages/trophies are nice for the "glory!" factor, but they're generally going to cost you resources, which costs you time and/or money.

That said, I'll try to cover a few types of attack that work well with the current meta, and what troops I personally prioritize:

• BARCH - this is the #1 farming strategy, hands down. You can throw in Minions and make it BAM if you want, but I find most of the time I get 80-90% of the resources out of a collector-heavy base with just barbs and archers.
• Giant/Healer/Wallbreakers - this is a decent strategy until around TH7 or so. By this point, bases are so large that giants really can't make it through the whole base, and the high level of ADs means that your healers won't last long.
• Mass Dragon + Rage - this is still a pretty good strategy for TH7-8, and maybe into the early stages of TH9. It's also really easy to deploy. I've tried being strategic before, and in wars I always find my cohorts who just drop a big mass of dragons get more stars than I do.
• Hogs - Hogs have been the subject of many nerfs recently, but they can still be effective if the base design permits it. Double big bombs and skeleton traps tend to do the most damage vs this strategy.
• Balloons + Minions (Ballonion) - This is a good strategy against bases where the AD is low level or improperly placed. You can also use dragons instead of minions if that suits you.
• Golem/Wizard/PEKKA (GoWiPe) - there are a bunch of high-end very high level DE troop strategies out there. I like GoWiPe, although I think GoWiWi (Golem/Wizard/Witch) is also popular. This is really for crushing bases that have a lot of stuff in storage, or clan wars if there's some really good reason to invest this much in resources.

Again, here, what I'd suggest is practice a new strategy, and if it works for you, then upgrade the troops. I'd also suggest upgrade everything before you bump your TH up, because you never know what the next meta will bring to previously useless troops. It's almost always easier to raid and save up for your upgrades at a lower TH than at a higher TH.