UPDATED October 26, 2014.
Shub has tons of monsters, a couple faction-specific beasties (Dark Young and Cthonian), the best "tutor" effects in the game, the best location destruction in the game, the best cost-reduction and resource-ramping effects, and effects that recur characters from the discard. It is an excellent faction.
1. Shocking Transformation (Core) - This searches for any character in your deck that you might want and/or need at the moment and pops it into play, all for sacrificing a character that you probably will get back anyway through some other effect. It's only downside is that it can't get an AO.
2. The Festival (TkatG) - Best card in the game? Definitely pre-restriction, but still a contender even post-restriction.
3. Burrowing Beneath (Core) - Probably the best support destruction in the game.
4. Black Dog (WoP) - While not the best in terms of efficiency, the ability is a very good surprise, and afterward forces your opponent to play differently once he knows you have it in hand. (see the Neutral/Colorless top 10 list).
5. The Three Bells (TUP) - Probably the cheapest and best sacrifice effect in the game, it forces you to sac one of your own, but again, you can probably get it back whereas your opponent might not be able to. It's just a bonus that it can put a character into play after 3 sacs.
6. Hungry Dark Young (Core) - This gives you 2 characters for the price of 1, which is great if you have sacrifice effects or that 2 cost character has an enters-play ability.
7. Y'Golonac (Core) - One of the cheapest and most efficient AOs in the game, he can suck the life out of weenies, attack and defend, all for a 1 cost effect.
8. Grasping Chthonian (IotF) - The only thing better than destroying a support card is getting a decent character at the same time.
9. Book of Iod (ER) - Excellent draw effect, probably better than many of MUs draw options, and it puts a card in your discard (where you might want it incidentally).
10. Savio Corvi (TiV) - While his abilities are both conditional, they are both fantastic.
This list was hard, since Shub has a lot of other good cards. But here are some honorable mentions:
- Twilight Cannibal (TH)- Number 11. One of the most efficient chuds in the game who sports the skill and icons of a 4 cost cards, but costs 2. Since Day isn't very much played right now, his downside isn't that bad.
- Snow Graves (AtMoM) - (see the Neutral/Colorless top 10 list). Could go in every deck, and shuts down a lot of deck strategies that they are forced to deal with.
- Nug (SoK) - This beast of an AO, through various combos, can get you through 6 turns of resourcing in one or two turns. He also has a great icon spread.
- Under the Porch (THBtS) - The same price as Shocking Transformation, but it can fetch an AO. It returns to your hand, but with Shub's resource ramping, you can play it again within a couple turns.
- Shub-Niggurath (TC) - recur all those dark young that you lost one way or another. Can you say swarm? GG.
- Feed Her Young (WaB) - A great card especially in a mono build, to get out ahead of your opponent. Bigger domains means bigger dudes.
- Corrupted Midwife (ER) - She is a great way to get back key characters or get back characters with enters-play abilities like Hungry Dark Young (Core).
- Ya-te-veo (TUP) - cheap, good icons, and a mini-Shotgun (Core) built in.