If you are happy just sticking to Birmingham and the West Mids metro area then you dont need a WM Day ranger, there is a much cheaper DayTripper ticket. (obviously if you are also using the DR to get there in the first place, then ignore that)
In answer to your question, all depends what you are looking for.
For sheer volume you wont beat New Street, although it can be tiring keeping up with all the terminating trains at both ends, especially with the crystal maze style new station layout.
Moor St far less busy, but interesting to see the old GWR style furniture. Ignore Snow Hill, you'll see nothing that you wont see at Moor St (except Trams) but miss all the Chiltern terminators, and its a far less nice station. I believe it was a car park for a long time before reopening; that seems to the the ambience they are going for post reopening!
'Interesting movements' sounds more like freight, Light engines and random stuff, which youll get none of at New St.
Agree with others on Stafford, Nuneaton, Water Orton, Bescot. I'd add Tamworth and Crewe to that list.
My top pick would be Nuneaton, busy, nice vibe, safe, plenty of trains calling, good variety, plenty of staff, shelter, tolerant of spotters. Theres something about the 5 way junction and flyover that just makes it... interesting.
Stafford is almost as good, again busy, safe, plenty of trains calling, good variety, shelter, also 'spotter friendly' afaik
Water Orton is a bit desolate tbh, I think its unstaffed, very few trains stop and no facilities. Safe enough afaik, but not the best bet, especially in Winter and if you are young and/or on your own (sorry for assuming!!)
Bescot, does have yards around and a ticket office, but little shelter. Much quieter than the others, you might get lucky but maybe not (check RTT). I'm probably biased bcs I remember the days of BS depot and yard siding with 40+ 20s,25,31s,45,47s,56s back in the day; so it seems like a wasteland to me now.LOL
Tamworth is theory is good, problem is the only places where you see both lines are the top of the two open staircases, with no shelter or seating. Id save it for the summer tbh
Crewe is a railway centre second to none, with all sorts of yards/depots around, some visible from the train or the station; you can also take a 15 min walk and take look at the the DRS depot, you can usually get 3-6 numbers from the approach road. It also has more variety of passenger stock, with 3 TOCS you wont see in any of the above (excpet NS): TfW, EMR, NT and a SIX way junction!
The one massive drawback is that most of the freight bypasses the station on the avoiding lines. You can see that from a bridge/car park outside the station, but it a bit of pain to get there and youll miss other stuff doing it. Thats why a lot of spotters prefer Stafford. All in all I'd say maybe the chance of seeing 'something unusual' is probably higher there than anywhere else, but you can also have bad luck and see nothing 'interesting' for a few hours!
If you fancy Crewe AND Nuneaton then there are hourly LM Trent Valley 350s between them. Southbound there is also a handy move on the Crewe>Stoke>Stafford stopper, few mins change at Stoke for the AWC Picc>Eus 1st stop Nuneaton.
Finally, for something a little different you could check out the unique Class 139 on the Stourbridge shuttle, then recover from the excitement with a pint in the superb Seven Stars at the bottom of the Stourbridge Jn approach road.
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Hope that helps, good luck and let us know how you get on!