Which is the best primary school in Tanah Merah?

That's hard to answer since I've only had my kids in the one school.
Loganholme state school is close and apparently good, but I've only heard from a couple people about it. It doesn't have a high school though I think, so you'll be looking for another school after primary.
Shailer Park state school is amazing, but you can find out yourself from going to all the peek at prep days, the open days and events held at the school. The teachers are lovely and incredibly dedicated to the kids. The high school is directly next to the primary and ranked top 3 above even John paul. I went to the High school and enjoyed it, but it's been years so things have changed, I think for the better though.

But it's better you find out yourself since I'm clearly bias.
Just check all the extra curricular they provide and look up teacher reviews. If the teachers don't like their own school, then I would advise not going there.