I haven't seen a ranking of the promotional cards yet, so I thought I'd start one.
#5: Walled Village
Yawn. Unlike most of the Village + a bonus cards, the Walled Village's bonus is something you don't want to take advantage of. If you're regularly putting your Walled Village back on top of your deck, you shouldn't have bought a village in the first place. It's nice when you get a bad draw, but most of the time there's no reason to prefer this village over the vanilla version.
#4: Stash
Stash's mechanic is a fun novelty, but in terms of gameplay it's only mediocre. Stash is usually just a Silver with the ability to show up in groups. Masters of deck reshuffles can certainly exploit Stash, and it makes the Chancellor a much stronger card. But, your opponent can also take advantage of your Stashes with deck attacks. Stash, like Silver, is usually not a bad purchase, but you hope for more at the $5 level.
#3: Black Market
Black Market adds a lot of variance to the game. If you're lucky enough to get a curser/VP chip/village when none of those cards are present in the normal kingdom, you're in great shape. But, most of the time Black Market is a terminal Silver, and if your opponent happens to get the King's Court out of the Black Market: gg.
In some Kingdoms, Black Market's ability to play Treasures during your Action phase allows some crazy hijinks. Tactician is the obvious example, but cards like like Quarry/Workshop and Library-type drawing also benefit from Black Market.
Playing Black Market in real life does add the mess of setting up the Black Market deck, but overall Black Market is a fun card and a middle-of-the-road buy.
#2: Envoy
Envoy sets the baseline for Big Money in Province games and shouldn't be overlooked in any setup when it appears. Why does it only get a #2 ranking? Well, Smithy is almost as good in most decks. Envoy provides a little more interaction and can sometimes offer difficult dilemmas (do I let my opponent draw the Province and hope he doesn't have a Tournament, or do I make him discard the Gold?), but there's enough overlap between the two to knock Envoy out of the top spot.
Which leaves…#1: Governor
Governor hits the sweet spot of flexibility and strategic choices. None of its three options are bad, but making the right choice in a particular situation requires not only keeping track of your deck, but also your opponent's deck. Gaining a Gold is powerful; is a Silver going to give your opponent too much of an economic boost or will it clutter his engine cards? The drawing option is a non-terminal Smithy, but giving your opponent an extra card is a steep cost. Finally, the remodel effect can create some insane endgame turns, but it can backfire if your opponent turns $4s into Duchies or takes an early opportunity to trash cards.