Here is what you should be looking for:
Reliability: they need to know what they’re doing. Favor providers with proven experience validating on the Elrond Network, at least on the test-net.
Communication: you’ll need to get information and support from them, on a regular basis. Be sure to pick a team that speaks a language you are comfortable with and matches your availability.
Economics: go for a competitive fee. While in theory “the lower, the better”, avoid 0% fees - validating on the Elrond Network is not easy, nor cheap.
You can find the official Elrond Staking Providers in the “Staking Infrastructure” section of the community-run website Elrond Partners: https://elrondpartners.com/ or you can select one of the providers featured in the app.