What type of sealant should I use for this? I'm assuming 100% silicone. 
Electrical – Sealant for external outlet
caulkingelectricalreceptaclesealant
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Best Answer
Actually 100% silicone isn't a great caulk in my book. It's hard to get off later and typically isn't UV resistant.
What you want is a solid outdoor oil-based (mineral spirit cleanup) caulk (sometimes called Window/Siding). I really like Quad, but anything made for outdoors will do (even some latex products).
If I were you, I'd also replace the cover. It's looking weathered. Once you've caulked it, you'll be less likely to replace it in the near future. I would highly recommend an in-use (bubble type) cover.