Your objective will determine what the best way of sweetening is. Most folks will attest that adding sweetness to wine helps to unlock the fruit flavors in the wine, so if you feel like your wine just doesn't have that fruity flavor you are looking for, do some bench trials (tests with small quantities of wine). Pull out a glass and taste it, add sugar in small, measured quantities, tasting each successive addition along the way. If you find that you can get the wine to your liking with just sugar, note the level that you like, do the math, and adjust the whole batch. This type of sugar only sweetening can be accomplished with table sugar or simple syrup.
Adding concentrate will have the same effect as sugar sweetening (there's lots of sugar in the concentrate), and will also add flavor and body to the wine. If you have a wine that's just not up to snuff, even after sweetening trials, concentrate may be the way to go. The downside is as you have noted, you'll have to clear the wine again if it's already clear. Don't know what kind of wine you are making, but if it's fruit wine, and consistently needs more flavor / body, consider increasing the amount of fruit per gallon you are using. If you are messing around with cheap wine kits, try making them to 5 gallons instead of 6, which will increase the body and flavor.
Having said that, and noting that you said you want your wine to be the best that it can, there's not a set in stone answer to your query, "what's the best way", as it will depend upon each batch and your personal taste. The best method, in my opinion, would be as follows:
Make your wine and clear it.
Taste your wine and do sugar / simple syrup trials to see if it's getting better, determine the best level
Do some concentrate addition trials to see those results
Make adjustments to your wine according to the most favorable trials, and if it's adding concentrate, then just suffer through the additional weeks of clearing before bottling.