EDIT: The below was me totally misunderstanding your question, I blame it on lack of coffee. Since I typed it I’ll still post it in case some bit is helpful. I’ll include brushes at the bottom. END EDIT:
That’s a very complicated question… the answer is both simple and complex. Akin to how do you put out a fire. You spray water on it right?..well sometimes.
Brands: I totally dont 
work for one of the bigger paint distributors… I’ll say this. So long as you stick to a bigger company and buy the quality products (EVERY manufacturer makes some products that are crap) then you should be ok. Porter paints (Now owned by ppg), Sherwin Williams, Benjamin Moore, all have good reputations in the industry, and more importantly will stand behind what they sell in the case there is an issue. I’m not saying that you cant get a quality product or good service from a mail order place, but you cant go in for coffee, BS with the tubby long haired guy behind the counter, and pick up another 5 gals at 6pm on a friday to finish up the job because you way under estimated the product you were going to meed.
Stains: Talking about stains is like talking about religion, it can get heated, and in the end no ones minds get changed. For decks I really like the look of the oil based toners, they stink, are a fire hazard if you dont handle your trash properly, but they can make a deck glow. And in my opinion dont lap nearly so bad or raise wood grain like the water based. The water based are low VOC are available in a ton of colors, and some can even be put on damp wood (I still dont recommend that personally give a deck 2 or 3 days to dry). Know that there are different finishes designed for vertical and horizontal surfaces. My preferred product for a pine fence is blended with a form of polyurethane and will look good and last a long time…ON A FENCE, you DO NOT want to use it for foot traffic on a deck, decks have their own coatings. That are more flexible.
Paint: in this field for exterior work latex is king and what you will run into 99% of the time unless your working on 1960s and before houses. If you run into a paint you aren’t sure of you can do a solvent resistance test. Take denatured alcohol (or NON ACETONE) Fingernail polish remover, saturate a cotton ball and rub an out of the way clean section of paint, scrub a spot the size of a quarter for 30 seconds, if you take paint off the surface or you get the color of the paint on your cotton ball,then most likely you are dealing with latex. If it doesn’t come off then it’s probably oil or one of the high tech coatings like kynar or some resin based coatings (Refer question like that to your paint rep who you can get as a professional at places like Sherwin, PPG, … ) you are more likely to run into oil or speciality coatings on metal, like roofing, sheet steel, window frames, some gutters. On wood or god forbid painted vinyl ( Seriously stay the F away from painted vinyl, and do not let your wife talk you into painting yours) you are almost positive to be dealing with latex.
EDIT: Now to answer your real question…
Pretty simple Buy Purdy, Wooster, Corona for latex paint and other water based coatings a medium stiff nylon polyester blend brush will do 90 percent of what you need. If you need a super smooth finish on something pick up a purdy nylox it’s super soft and I dont know if you could make it leave brush Marks. For oil I only use Natural bristles brushes, Purdy, Wooster, Corona all sell great natural bristles and will all do the job. For decks wider and thicker is better, and again if using oil stain the natural bristles will hold more and leave a better look BUT you have to take care of them.
Anything I can clarify shoot me a msg.
