Very honestly, it all depends on the other flavors being used in a mix.
Just off the top of my head...
In general, creams and vanillas will add sweetness and round out a mix. That depends on which ones as some are sweeter than others. Creams and vanillas will also pull mixes together just like they do in real life recipes. Creams will also add body and mouth feel.
INW Shisha Vanilla has some sweetness. It's on what I call the brighter and lighter side of vanillas. FA Vanilla Tahiti is another bright and sweeter vanilla best used with milder fruits. FLV Cream will bring some sweetness, body and mouth feel. As little as 0.25%-0.5% of those will influence a mix. Several of the frostings will bring sweetness. FLV Whipped cream will bring sweetness, body and mouth feel (in as little as 3-4 drops to 30ml, careful it is strong).
CAP Vanilla Custard v1 and FA Custard Premium are sweet.
I'll give you a good example. If I have a really tart raspberry profile and I want to bring in some sweetness I will use a little FA Juicy Strawberry, or another s-berry, to sweeten up the raspberry. And you won't taste the strawberry when it's used lightly cuz raspberries, being a darker berry, will overpower the s-berry.
Slightly off topic - adding a drop or two of a raspberry to a strawberry mix will enhance nearly any strawberry. Same with adding a light touch of a watermelon to a strawberry.
Another example that I use. Using FA Pear adds a creamy sweetness that’s perfect with peach or apricot whether those are FA versions or not. It kills off some of the astringency that some peaches have. And it pulls everything together nicely. FA Pear gains creaminess as it steeps. Used lightly, you won't notice the pear at all.
FA Jamaican Rum is sweet (more sweet than boozy, think butter rum not “yo-ho-ho”). I've use it in very small amounts when I wanted to make a mix a little sweeter.
Using a touch of FA Walnut will add a mild sweetness to a mix. It will also contribute buttery notes when used with creams and caramel/butterscotch. FA Almond with a light touch will also sweeten up a mix (depending on other flavors used).
FA Anise, lightly, will brighten and sweeten.
Some oranges will sweeten and brighten up a zesty citrus mix.
The sweeter caramels and/or butterscotch flavors, used lightly, will sweeten up a bakery mix.
Some of the really sweet cookie flavors, used very lightly, will sweeten up a mix.
FA Apple (Stark), used lightly, will add sweetness while adding no discernible apple taste. FA Torrone will totally sweeten a mix, but it is strong, I've only used it in drops in a 30ml.
All of those ^^^
depend a lot on the other flavors I am using in a mix. There are many more. What I reach for has a lot to do with the other flavors I am using.
There are many ways to bring some sweetness into a mix and to help round it out.
Sorry
that got longer than I initially planned on.