If the dealer has 2 to 6, it’s a fair chance that he will bust, so best thing would be to stand and just hope for best. In all other chances, the dealer generally has a better hand then you and even though the chance of busting is high, you need to take a hit in order to do anything. Hard 17 to 21, as mentioned is always the hand to stand. Plain and simple, chances to improve and get anything better, are quite low, and even 17 is not a bad hand, so just sit back, relax and hope for best.
Soft hand of 13 or 14, is a really solid hand, gives you some interesting options. If the dealer has 5 or 6, then you should double down, rest of times just take a hit. You should never stand on this hand, because the total itself is useless, and worst case you get a 10 and your score remains unchanged, while every other card improves it. 5 or 6 also give the dealer a good chance of busting so it’s a good idea to invest some more money into it.
If you get a soft 15 or 16, and the dealer has 4, 5 or 6, you should double down, if the dealer has anything else just take a normal hit. Similar to the previous case standing down is out of the question, since this is not a very viable total. Doubling down is a good option if the dealer is likely to go bust, and since there is no chance of going bust, only on improving, this is generally a quite solid hand.
When you get a soft 17, again stand is out of the question, since there is no chance of busting, and the worst you can get is 17, beating most of the dealer hands. So in case of dealer having 3, 4, 5 or 6 you should double down, and in other cases just take a regular hit. Having a soft 18 is somewhat tricky. Again no chance of bust, plus the potential hand of 18 is quite good, but the downside is if the card is not a ten you have the potential to actually hurt your hand. So while having the advantage you will want to put some more money in the game, therefore if the dealer has 3 to 6, you have a very good opportunity, as he has a good chance to go bust, while you have none, plus you also have a good chance of improving your hand, in that case you should double up. In the dealer has a 2, 7 or 8, best thing to do is stand. Dealer card of 7 or 8 lead to a possible 17 or 18, which means hitting will most likely lead to hurting your hand. Dealer hand of 2 means less likely to go bust. In case the dealer has 9, 10 or ace, you again assume he actually has a 19, 20 or 21, which means he has a better total than you, and you need to improve, so in those cases you should take a hit.