to use a quirky analogy that betrays my wargaming days from childhood - you might carry a 2d10+1 morningstar (for the non-geeks - that's 2 swings doing 1-10 damage each, plus one
) and a 4d5 longsword. both average 12 damage overall - but the damage profile is different. the morningstar is more prone to hitting extreme highs and extreme lows, whereas the longsword will do more reliable middling damage. you also carry a magic staff that shoots an icebolt for a guaranteed 10 damage and always 10 damage, but it only has a limited number of charges.
sticking money in the bank is like the icebolt. guaranteed return, but in the long run, below the average performance of the other alternatives. and you can't keep doing it in my opinion (just like the limited uses of icebolt) because if you keep all your money in the bank for too long, eventually you get eaten up by inflation.
low beta/more stable stocks eg. WOW (this is not a recommendation - i am just pointing out an eg. of a low beta stock) is like the longsword. better returns than sticking it in the bank, but unlike sticking it in the bank, there is a chance of a subpar return.
high beta/volatile stocks eg. LEI (again not a recommendation - just giving an eg. of a high beta stock) is like the morningstar. gives you a (theoretically) better shot at a big gain, but also exposes you to potentially bigger losses. possibly even crippling ones if you haven't managed your risk properly.
so going back to adventuring - the attack you choose depends on the situation you're in. if you absolutely HAVE to take out the evil dark sorcerer NOW, before you and your buddies get horridly wilted or some other nasty fate, and you need to do a truckload of damage in one round, then take out your morningstar and hope for the best. if that dark sorcerer has already been injured down to 8 health by one of your buddies, it's your attack and you're the last one to act before the bad guy can counter, you'd be crazy not to burn one of your limited icebolts, even though both your martial weapons have a good chance of hitting for 8 or more, because if it doesn't, you're toasted!
but if you're facing a horde of puny goblins where each individual one will morale break and run away if you just nick it for a few points of damage in a round, you whip out your sword, to almost guarantee you'll break one every round and thus thin the crowd so you don't get overwhelmed by numbers. no need to burn your limited icebolts, even on the off chance you don't chase one away this turn and it hits you, no real harm done, until it happens a few times and you get low on health.
the whole point of that is, what you should choose to invest in is SITUATIONAL. if you absolutely cannot afford to lose any money at all for a given timeframe, you probably should leave it in the bank. if you're comfortable for the moment and can shoot for a better return, but you can still live comfortably if your investment goes moderately down in value, then low beta stocks may be more suitable. but if you have a bit of cash whereby if you lost most or even all of it, it wouldn't really affect your quality of life, and you can sleep at night even knowing that there is a possibility of that happening, then you might be more willing to take a big home run swing at something and go for the higher risk stuff.
i don't think there is a "best" stock. the best stock for my purposes may not be the best stock for yours. i might have gotten an unexpectedly nice bonus (as if - that hasn't happened since 2007!
) and i might be quite happy to punt it all on some volatile stuff, whereas you may not feel comfortable with that sort of risk.
what you should invest in depends on your goal, risk appetite, tolerance for volatility, and your current financial situation. only YOU know what those truly are.