We will be in Munich end of August and deciding on which hotel : Le Meridien (seems convenient location across from train station), Westin Grande or Munich Marriott. Does anyone have reviews on the above? We want to use our loyalty points so that's why staying at this brand.

What are you after? Depending on what you plan to do, Le Meridien is closest to the center, if that is your interest. I have never thought Le Meridien was in the same class of hotel as the Marriott or the Westiin brand. If you are interested in heading out using trains, this location simplifies trips to Neuschwanstein, Dachau, Nymphenburg Palace, Herrenchiemsee, Salzburg, etc. There are many things within walking distance from there.
The Munich Marriott and the Westin are in the soul-less business area and are so far from the center that you need to take some kind of transit to go anywhere in the city perhaps except for the English garden from the Munich Marriott. They are short trivial rides, however. If your objective is to stay in the hotel, these are the hotels for you.
I was in Munich last month but stayed in the historical center. Of course, I did not have to be limited to a Marriott group hotel.

1. Meridien (for reasons explained by greg)

2. Westin (at least a decent choice of restaurants, eateries etc nearby as that area is also the "town center" and transit hub for NE Munich)
3. Marriott (easy access with car, and direct connection to airport by BUS - not train - are the only two upsides that come to mind)
The latter two both have easy access to the historic city center by subway in short walking distance.
Nevertheless, both neighborhoods are very "modern" and do by no means resemble the picture of Munich you find in your travel guides.

Le Meridien would be my choice but keep in mind that this is in a downmarket area for such a nice/expensive hotel. It's been a while since I've been by it but I used to note to myself that it seemed a bit oddly placed there. Not a big problem but the immediate surroundings might not impress you or be what you'd expect. It's not a bad/dangerous area (although a lot of transient types) but it's very close to the main station which is a busy area of cheap hotels, kebab shops, etc. I only mention it because a hotel's location is part of what you're paying for and while this gets top marks for convenience and general 'central' position, it does not score well on general appeal of the immediate area.

Thanks everyone-based on the above I think Meridien is the best choice for its location to the station-really, we will be out most of the days anyhow exploring the city so easy access to that is desirable. The only other hotel option within this brand is Roomers Munich which looks funky inside but not sure that location is any better?

Our go-to in Munich is the Marriott in Munich Ost - not in the center, but a short walk to the Ostbahnhof from where you can get anywhere by train - it's a much quieter area than the Hauptbahnhof.

Having said that, I've just booked a trip to Munich and have chosen to stay near the Hauptbahnhof this time, which I've not done for many moons.

Keep in mind that there's really no need to be near the main train station, as long as you're near a U-Bahn or S-Bahn you can get just about anywhere.