Try posting in the right section first.
Anyway, these are probably the only "musts" in terms of watching order:
- TV series before Reflection
- TV series before Movie
- Reminiscence before Reflection
Reminiscence was produced before the TV series, and chronologically it takes place before the TV series. However, it shares a similar tone to Reflection and what happens in Reminiscence isn't strictly required to watch the TV series or anything, so that's why I didn't count it as a "must" in terms of order.
I would probably do this:
TV Series -> Movie -> Reminiscence -> Reflection
You could watch Reminiscence first if you want, or the Movie last. But people feel mixed about the TV series, and generally almost everyone loves Reminiscence, so instead of feeling a letdown after watching Reminiscence, I'd watch it AFTER the TV series. The Movie is also not considered very good either, so that's why I put the TV series and Movie before the OVAs.