I actually have six.  Six only.  They cover all bases, all seasons, from my great Cary Grant unlined tan Brooks in the spring to the wool-lined fly-fronted 1967 Burberrys' in early autumn.  I have the Gloverall for now and until next March if need be.  Then there are two vintage Dhobi and an English Squire.  At our age, Woof, you can't afford not to be warm.

Great topic. but Simon Crompton's coat list is probably influenced by who has given him a freebie.

Due to global warming I think winter coats are as much now about rain and being light weight as they are about keeping warm.

The five I mix up are a Japanese fit Barbour Beaufort, Ralph Lauren Peacoat, a lovely longer length Black Watch tartan overcoat that was a special order from John Simons, a navy blue, half lined fly front raincoat with ticket pocket again from Ralph and navy blue Polo coat for more formal occasions.

I recently purchased a yellow short raincoat from John Simons. To be honest I'm not a fan of the short length but the colour looks great. Again, with wetter weather in Spring/Summer I think this will come into it's own.

Spendthrift, for what it's worth may I offer you a simple piece of advice.
Whatever you buy match your outwear to your own personal style.
I really hate to see guys who wear say a formal three quarter business coat with jeans and sweater on a Saturday or men in a suit where the jacket overruns a peacoat for example.
I tend to buy classic clothes that last and are looked after so I've been fortunate to end up with lots of different overcoat options but the one coat I wouldn't be without is my Barbour. I wear it with literally everything, it has been bundled up and used a pillow by my daughter on long car journeys and still looks pristine. The lightweight construction is ideal for the strange times we live in and the fact it's waterproof makes a big difference.
£ for £ these coats really can't be beaten.

Black Watch is ticking a lot of boxes with me at the moment.  My daughter is giving her a boyfriend a rather expensive black watch scarf for Christmas.  I felt, upon seeing it, slight pangs of envy.  An overcoat, however, strikes me as magical.  A special order?  Do tell.  As for those shorty raincoats, there was a fair amount of interest on here about twelve to thirteen years ago: the 'Harry Palmer' look? 
Woof's right about Ebay.  Some names command fairly high prices - Grenfell, I think, is one of them - but my Burberrys', bought in 2010, was no more than £30. 
It must have been a 'Golden Age' on Ebay around 2008/9.  I was buying a huge number of items from US sellers.  Not so now.

And if you were obliged to ditch one, which would it be?  Me - the English Squire.  Dhobi I believe to be underrated as an inexpensive vintage coat.  Seen once upon a time as a 'one-off' at Russell Street, they are fairly easily available on Ebay (and probably elsewhere).  Same kind of provenance as Baracuta when made in North Manchester.

Mm...  let me think now...  something muted down below...  colourwise - mid-grey?  Might need a 'hefty' shoe in order to carry it off.  I'd guess, given the right set of circumstances, a very restrained tweed - either 3/2 or even two - would work beneath.  White shirt, solid tie.  Knitted perhaps.  Black or navy?  Would need a fair amount of chutzpah to carry it off.  So, maybe a little limited. 
I'd wear a Black Watch jacket, given some nice detailing.  Maybe with chinos and sockless.  Loafers?  Probably. 
Nice debate going here.  What 'Talk Ivy' is all about.

A Black Watch board shirt or overshirt is something I would definitely wear. I would find a jacket too loud and look at me-ish. Trousers maybe this time of year are fine.

I have always been drawn to Black watch tartan. The palette is elegant and goes well with everything from jeans to evening wear.
The Christmas before last John Simons had a beautiful black watch overcoat.Unfortunately is was only offered in their shortie cut. I'm not a fan of this style as my firm belief in that overcoats should cover your thighs.
I was told that they had only made about 3-4 coats in this fabric but had enough for one more coat. I have a vintage full length brown tweed Parkes overcoat that was used a template. It's fly fronted with raglan sleeves and best of all it's knee length. The cut is still slimmer fitting so it avoids any baggy !980's styling.

RunningGeez probably knows all about Parkes.

Just remembered having a conversation with The Weejun, and we spoke about a shop in the Piazza at Convent Garden called S Fisher (a great shop) that used to sell Parkes corduroys.

Sam Fischer was in the middle of the arcade and specialised in cashmere and John Smedley. This is before Smedley had their own shops. I can’t recall when they moved out but remember in the eighties buying from them. This is the time  when the arcade was full of great independents.Not at all like it is now.You could walk down Bond Street to Herbie Frogg who sold beautiful suits down through the arcade for knitwear and up to Barons on Piccadilly who sold really great sports jackets, knitwear and Burberry raincoats.
Those were the days that you came to London for something you couldn’t buy in the provinces. Now it’s mainly ubiquitous tat that’s sold on every high street in the land.
I know this is a massive generalisation but London was choc full of great  men’s stores in places like St Christopher’s Place, side streets off Bond, Wodehouse on Oxford Street, a few dotted around Covent Garden and Grey Flannel in Marylebone .