IMHO
1) Crowns, Leaves
2) Factories
3) Castles, clocks
4) Lightbulbs
The thing to note about crowns, leaves, and factories, are that they're the only symbol with "I demand" dogmas. The trick to I demand dogmas is that they're worthless if you don't have more icons than your opponents; regular dogmas can still be shared, often very profitably, if you have fewer cards.
-Crowns have many attacks, and some powerful attacks, like Gunpowder, Pirate code, and Combustion, that can lock opponents out of the game, and score enough to end it.
-Leaves have fewer attacks, but a greater ability to interact with a large number of cards. Drunk Protestants (Fermenting-Reformation) is the quintessential leaf combo.
-Factories are powerful scorers and are the first icon you build up from 0. Because of the variance (there are 3 factory cards in age 4), the first person to draw one gets a lead that's difficult to displace. It doesn't hurt that factories tend to splay and tuck more factories (while sometimes destroying your other icons).
-Castles have very strong I demand effects, but there are only 3 castle dogmas out of the 4 cards with castle icons in age 3, and 0 past that. There are a ton of them, then suddenly, there are none of them and they're a liability on your board. You don't necessarily have to work to accumulate castles - it kind of just happens naturally. You manage castles. You exit out of castles. You generally don't hope to draw more castles.
-Clocks have no I demand effects. The game is often over before clocks come into play. I don't think I've ever deployed clocks to avoid sharing a clock ability. If you ever do need to "accumulate clocks" to avoid sharing/force sharing, it'll generally dictate a single decision, rather than an overall strategy. The main reason you will ever accumulate clocks is because it's tied to two achievements: 3 clocks for Empire, 12 for World. Note that even if you get perfect splays and every clock card, you cannot get 12 clocks before age 9.
-Bulbs have no I demand effects. What's more, about half of the bulb effects are situational or effects you don't mind sharing. One of them is even a kind of attack when shared (physics). What's more, a number of bulb effects are focused on teching and skipping ages. If you share bulb dogmas with a teching player, the extra draws will help you deplete the lower piles and nullify his teching advantage. Now there are a couple of bulb cards that are great and worth keeping to yourself, but I wouldn't work on building up a bulb lead to avoid sharing it. Your opponents don't have much reason to challenge your bulb lead in a vacuum.