Thanks.

I'm not much of a candy flav guy. Might give MB big watermelon and/or WF yellow watermelon a try.
I vape watermelon and other melon flavors here and there. If you are looking for a realistic watermelon go with FA Watermelon (Red Summer). It includes the fresh red, seed and rind. It is not sweet or complete on its own so you will need others for that. FA watermelon is not strong so depending on your other flavors 3.5-6% is not uncommon in a mix.
I make it more like a fresh tasting farm watermelon by adding a little MB Big Watermelon. That will bring the sweetness we're most accustomed to with eating a real w-melon.
MB's is a candyish watermelon. By itself it is like a Jolly Roger hard candy, sweet.
Also there is something about adding a tiny bit of raspberry to watermelon that adds even more realism. I use INW Raspberry WG for that but be careful because it is strong. Depending on %'s of w-melon used a single drop of INW raspberry wg will influence the mix.
Thinking this with creams/vanillas/custards (depending on %'s):
FA Watermelon 4-5%
MB Big Watermelon 1-1.5%
INW Raspberry WG 1 drop
ETA: I've heard great things about the realism of MF's watermelon (and many of their other flavors). They are very highly concentrated.
@Opinionated uses a bunch of MF flavors. Maybe she has w-melon and can offer her thoughts.