Same hear - plain 5% yogurt. Usually I put it on the side with avocado puree for lunch or supper.
In the am, I usually make up a bowl with a frozen fruit cube thawed first, yogurt, and a bit of cereal - but not mixed. Then some bites are single food, some combos, and she's always gone for the plain yogurt just fine (started at 7 months).
I accidently bought french vanilla instead of plain, and she sure did like it. She's had spoonfuls of flavored yogurt from my bowl too and gobbled it up.