Here's my favourite seat on these trains...
GWR Class 387: When forward facing, it's one of the 2 table seats with the disabled toilet in front of you and full (rather than half) window alignment. It also means no one can sit opposite facing you which means First Class-style legroom.
Class 80x and 810 trains: When it's travelling on non-electrifed tracks, any forward facing window table seat in a diesel engine coach. I've mostly been on GWR ones, 1 LNER example but no others yet but the motors and when in diesel mode sound lovely. If passing by the sea, then it's the same type of seat but on the sea view side of course.
GWR HST's: When forward facing, one of them is seat 69 or 72 in Coach C when they're seating layout changed when they were refurbished in 2006-2008. Plenty of window view but ridiculously generous legroom - I've been lucky 2 or 3 times when catching one back from London Paddington to Oxford. The sideway seats too - I remember me Mum and my sister back in like 2008 (I'd have been 15) were travelling back from Reading to Oxford after a day out. 15:50ish it was. Seats 69 and 72 were already taken so one of the sideway seats it was. I remember having a fairly quick read of my High School Musical book.
If I board a 4-coach GWR Castle HST and Coach A (the TGS coach) is at the rear, I'll make a dash for A73 (that single seat)!
XC HST's: In Standard Class it's seat G05 in the Quiet Coach. G03 which is the same side but in facing the same way as the single seat in the HST TGS coach - both are window table seats that are aligned with the window. G05 I'll be booking for the Birmingham New Street to Edinburgh leg when I get there eventually - backwards on the sea view side or forwards on the opposite but it faces the power car. It feels like an own-compartment Mark 1. I've not sat in G03 yet.
Class 165's: Complicated so I'll try and be brief, if I board a 165 at a station where it starts from (if I board at a buffered platform), I'll be lazy and board at the back of the train if Standard Class. This includes sitting in one of the back window-aligned seats with the cab immediately behind me.
Class 166's: One of the bigger window table seats at the cab ends, particularly the ones facing the cab - I know I'm not on my own with these table seats!
XC Voyagers: If / when I have a suitcase with me, I'll book seat F33 (window view table seat) when boarding at a calling station rather than starting - I'm in better view of my luggage which there's the overhead rack for backup. When I return from Edinburgh, I'll catch the train that leaves around 7am that goes through to Oxford without a single change of train unless there's a fault or disruption. I'll book seat 52 in Coach D (window view table seat) because...
1. I can put my luggage in the overhead rack.
2. To Birmingham New Street it's backwards on the sea view side or forwards on the opposite.
3. C28 and F33 is the same. But I'd rather leave Coach C for those that make a 10 (or last) minute seat reservation. I'd book seat C28 if D52 was unavailable.
Next time I board a Chiltern Class 168 - at a buffered platform, I'll board at the back and sit in one of the window table seats. I did this when I went to Bicester Village back in February 2017 (13:10ish from Oxford). Was disappointed it wasn't a blue and white 6-gears 172 strangely. At least the 168 was 3 coaches rathe than the 2-car 172.
Loco-hauled Mark 1 or Mark 2: Hopefully I'll be able to sit on the left hand side of the train in the front window table seat at the front of the train nearest the loco. I say loco-hauled Mark 1 as there's Mark 1 DMU's and EMU's.
Say a Class 101 when I catch one: A rail enthusiasts favourite window seat when forwards - I think we all know where that would be for us!
Pacer: Where possible and when forward facing, always a window seat in the front coach nearest the front - I've heard of diesel fumes coming in on these trains so avoid it where I can.
TransPennine Express (TPE) Mark 5 when I catch one in First Class: E32 - bigger table seat by the window with the kitchen right behind you! I've seen the online seating layout on the TPE website. I'll be able to hear the Class 68 noises better!
TPE Class 397 when I catch one: Literally any window table seat as these are fully window aligned I believe.
TfW Class 150: When forward facing, it's the window table seat on the left hand side nearest the drivers cab. If I'm ever on a 150 instead of a 175 any time then this will be a great seat.