Some kind of striped button down shirt, repp tie, blazer, extra-long cuffed cords (no high water pants), and argyl socks with probably heavy brogues.

The blazer would have to button the top button and be extra long in torso length, plus made from some heavy material (not summer wool).

Shoos would be brown wingtips.

Belt leather but not matching the shoos in color.

Long hair, windswept.

No jewellry or watch of any kind.

White OCBD with a pale grey bengal stripe. Standard Brooks supima model in either tradd or slim-fit would do me fine.

Black silk knit tie, 2" to 2 1/2" wide.

Navy sack blazer, either navy seersucker with bone buttons or plain hopsack with brass. Patch/flap pockets below, patch pocket on chest. No pocket square. Either way, early 60s cut with 2 1/2" lapels and a high roll (bandofoutsiders describes it perfectly, it rolls directly over the top buttonhole). Two sleeve buttons, welted seams and oh fuck it a hook vent while I'm there.

Slim uncuffed lightweight mid-grey flannels.

Black belt with engine-turned silver buckle. The buckle I would have inherited from my grandfather.

Green socks. I have a thing for green socks just now. Falke are nice. No argyles.

Black Weejuns, just in a better leather than the contemporary ones.

Old Seiko Bellmatic on a NATO strap.

Hair once more.

I'll be the first one to commit a little heresy. My ultimate look wouldn't include a coat or blazer. I like sack jackets and wear them but my ultimate goes like this: Grey or Navy Cardigan, white oxford shirt with nice collar roll, worsted wool pants or even teflon cotton(something that holds a crease), gunboats or plain toe bluchers, socks the same color as the cardigan, tie optional but if so inclined then a repp tie or wool plaid tie. Hamilton field watch on subdued NATO strap. Pretty boring but I don't think it gets any better than this..

Harris  tweed  jacket
white Mercer oc  bd w/Press style flap pocket
3" stripe tie  B2 c 1980(I purchased new)
grey flannel  trou from LL Bean
weejuns-cordo/more or less

interesting posts  and  great differences  in interpretation which is to be welcomed.....

Ultimate Ivy is surely a toga and a keg. They're not really me though. I won't name a whole ensemble, but a few favourite combinations:
Tweed jacket and a candy stripe OCBD (herringbone with blue & white probably my favourite, muted colours, patch pockets)
For summer, a cotton jacket and a pinpoint BD
For a more casual look, a plain heavy sweatshirt and white trainers (Keds, Chucks, Purcells...)

It's too hard to name ultimate trousers. I like all the obvious choices equally. As for shoes, I prefer longwings to loafers.

3/2 sack jacket, with slim lapels and a tipped roll, in light grey herringbone tweed, with three patch pockets, lapped seams, swelled edges and hook vent. Pale blue Brooks OCBD. Slim silk repp tie, which includes dark blue, red and yellow stripes. Tapered flannel slacks, no break. Proper weejuns.

Or, the same jacket/shirt/tie combo, but as a three piece suit including a four-pocket, six-button vest, and with Alden shell cordovan long-wings.

I know one of you lot blagged one of these not so long ago...... I could be envious, I really could.

Listening to 'Monk's Music' as I type...  Hawkins and Coltrane blowing...