Secure!? If you are talking about uncompromised apps from the author, then I'd guess Googlestore is the place to go - understanding that the code/application could be hacked on the author's site after hand off to Google (e.g. source code says one thing, executable says something else). To guard against this, you could download the source, inspect it, and compile it yourself (e.g. As routinely done in "source distribution" used in some Linux distributions).
IIUC, F-droid inspects and re-compiles the source, removing nasties in the process.
- Naturally there would be a delay in getting an F-droid app when compared with either Gstore or Github (or Aurora).
- F-droid tweaked and then compiled it; of course they should sign it.
So if by Secure, you're referring to an app more likely to be Trojan free, I'd go with Fdroid, in spite of the signing criticism.