If you can find any remaining stock of a Shakedry jacket then that will be smallest by a lot (Gore/Rapha/7Mesh all similar, Castelli Idro normal version also ok but Idro Pro has lots of stretch panels and doesn't pack as small). If it's not intended as primarily a rain jacket then probably ok to buy secondhand too, but most of them end up with little tiny pinholes in them once they've been scrunched into pockets a few dozen times during normal use, so they'll be a decent riding waterproof but not outright water*proof* anymore.
I'm not totally au fait with the current crop of replacements but at the tail end of the shakedry era I bought and returned quite a few whilst looking for the ideal solution for me. Here's what I ended up with in the end and why:
- Gore 1985 (or C5 - viz version for me) - bikepacking waterproof - tiny pack size but fit is generous enough to go over a slim down jacket in a pinch, and both front and tail are long enough to cover said down jacket. Flaps around a bit in the wind due to the same fit parameters so not ideal unless the extra coverage is needed.
- Gore Race (or C7) - slimmer more aero fit but didn't pack as small. Returned due to fit preference with another brand but was a solid contender.
- 7mesh Oro - returned because I'd already missed prime availability of my size, but I think was the actual lightest/smallest of the lot.
- Castelli Idro Pro - far from smallest pack size as had lots of stretch panels which are bulky, but seemed like the most comfortable/aero fit for me. Kept it for a few years thinking it would be my 'ride in rain all day' jacket but then found I actually preferred something like a Gabba for those conditions. Just sold it BNWT on marketplace for a modest loss.
- Castelli Idro (normal) - returned due to uneffective compromise between stretch panels, fit, and pack size for me.
- Rapha Pro Team - I missed out getting my hands on one of these but I now have one of the Gore Active successors to it which is the same fabric pattern/fit I think. For me this was the sweet spot in terms of a roadie waterproof - fit is more aero than the Gore 1985, no stretch panels to bulk it up like the Idro Pro or the Gore C7, and slightly fuller-featured than the 7Mesh. Does not have enough coverage for off-bike bikepacking use keeping an insulating layer dry underneath, but even the 2-layer Gore Active version I have now packs down to almost exactly the same size as the more generously-fitting Gore shakedry C5.
I know I've just given detailed description of a lot of discontinued/unavailable products, but I think
most of those brands kept their shakedry garment patterns alive in successor products, most of which are using 2-layer Gore Active like the Rapha that I currently have. Not sure what product names the successors picked up but shouldn't be hard to find.
To put those in order from smallest pack size to largest (for the same fabric type), I
think they were:
- 7mesh Oro
- Rapha PT
- Gore 1985 C5
- Gore C7 Race
- Castelli Idro
- Castelli Idro Pro
And in order from most aero fit to most coverage:
- 7mesh Oro
- Rapha PT
- Gore C7 Race
- Castelli Idro Pro
- Castelli Idro
- Gore 1985 C5
Also pretty confident that I ended up being a Medium in all of those, so that should be comparing sizes like-for-like.