My opinion on the 10 strongest helpers in the game. In no particular order....
Builder / Oil Miller / Farm Worker
Cards that let you play extra cards are the strongest. Getting an extra card per turn will put you ahead of your opponent right quick.
Peon / Merchant
An extra revenue die per turn is absurd, and most of them are close to the VP in raw value anyway, and if you play them right, 1-5 will always outpace the extra VP. The 6 is of course weaker, but still, Merchant is exceptionally strong on any turn except for 6.
Migrant Laborer / Long Distance Trader / Peddler
These are game-altering cards that can effectively change the way you play the game, in one way or another. These are strong, but are more difficult to use properly, as opposed to the absurd power of the above 5.
Packer
An innocuous card, but a simple free ship per turn is much better than it sounds. There are many cards that give you an extra shipment to either a barrow or building, but they are all restrictive or require you to use a certain die. Packer has no restrictions.
Beekeeper
The strongest money maker in the game, with a [fairly easy to achieve] 3 silver per turn is very strong.
Other contenders:
Wagoner is basically a sure 2 silver per turn with no restrictions. It's much easier to get value out of than Beekeeper, but the ceiling is lower.
Agricultural worker can be 4 silver per turn, but so many times you need that grape back anyway. You really need two grape fields for this to be worth it.
Joiner hits the top 10 in one and only one situation: Turn 1 with any extension that has a pig spot. Then you can breed pigs on the first turn with your starting commodity.
Roofer is also good in only one specific situation: The deli is open, you've got a 2-good barrow, and you know for sure you will get the 2-good roof. That's a lot to ask, bit if it's there, you can drop your barrow at basically no cost, and very easily get deli.
Builder / Oil Miller / Farm Worker
Cards that let you play extra cards are the strongest. Getting an extra card per turn will put you ahead of your opponent right quick.
Peon / Merchant
An extra revenue die per turn is absurd, and most of them are close to the VP in raw value anyway, and if you play them right, 1-5 will always outpace the extra VP. The 6 is of course weaker, but still, Merchant is exceptionally strong on any turn except for 6.
Migrant Laborer / Long Distance Trader / Peddler
These are game-altering cards that can effectively change the way you play the game, in one way or another. These are strong, but are more difficult to use properly, as opposed to the absurd power of the above 5.
Packer
An innocuous card, but a simple free ship per turn is much better than it sounds. There are many cards that give you an extra shipment to either a barrow or building, but they are all restrictive or require you to use a certain die. Packer has no restrictions.
Beekeeper
The strongest money maker in the game, with a [fairly easy to achieve] 3 silver per turn is very strong.
Other contenders:
Wagoner is basically a sure 2 silver per turn with no restrictions. It's much easier to get value out of than Beekeeper, but the ceiling is lower.
Agricultural worker can be 4 silver per turn, but so many times you need that grape back anyway. You really need two grape fields for this to be worth it.
Joiner hits the top 10 in one and only one situation: Turn 1 with any extension that has a pig spot. Then you can breed pigs on the first turn with your starting commodity.
Roofer is also good in only one specific situation: The deli is open, you've got a 2-good barrow, and you know for sure you will get the 2-good roof. That's a lot to ask, bit if it's there, you can drop your barrow at basically no cost, and very easily get deli.