Many vanillas I've tried from eliquid shops leave flavoring sediment behind, clogging the stuffing. So my favorite is Vapor4Life WOW Vanilla which is an odd vanilla, more like marshmallow and cotton candy but it's sweet and I like it. I also enjoy their WOWbacco which is a cocoa tobacco (hardly any tobacco flavor to me). At least sign up there so you get their sale emails. Their shipping is high (around $7) but if you catch the sales it makes up for it.
Also - Gourmet Vapor has a 50% sale until the 14th and I like their vanillarum (a customer creation) - it's vanilla w/ caramel and rum and it's really interesting. I would buy 80PG/20VG since I only use thinner juices in cartomizers. They don't have sales often so now is the time to try some juices from there. But if you create any of your own flavors, I recommend you mark them private and then if one comes out good, go back and mark it public if you want. I do that so others won't be buying my mistakes.
Another nice flavor that leaves no sediment is FreedomSMokeUSA Pomegranate. All of the FreedomSMokeUSA FlavorRange juices are available in any PG/VG (and I would get 80PG/20VG or even 90PG/10VG for cartos, but you could try 70PG/30VG too since Volt cartos have looser stuffing than the ones I usually use and they may be able to handle thicker liquids). oRYental 4 is nice too - it's in the tobaccos but it's more like a sweet caramel w/ a kick. FreedomSMokeUSA Specialties and Samples are only available in 50/50 and that's thicker than I use in cartos. I drip a few 50/50 juices on an atomizer (like Hopscotch and Voodoo). Some day I might thin one with a bit of distilled water and try it in a Volt carto.
Just FYI - PG is thin and lets through the most flavor, nicotine hit and nicotine irritation. VG is thick and dulls everything PG lets through but adds thicker vapor.
Some other interesting shops are Vaporbomb, Velvet Vapors, Juicy Vapors, Purevapes and for DIY juices see the kits and supplies at OneStopDIYShop and DIYFlavorShack.