1st of all, I wanna make a casual, but highly potent, ritual deck.

My first question is, what are the Top 8 ritual monsters?

Second of all, what cards should i put in the deck, besides and mons and their rituals, manju (looking for further ritual support)?

IMPO top rituals out there are:

- Relinquished
- Paladin of White Dragon
- Elemental Mistress Doriado
- Dark Master - Zorc
- Reshef the Dark Being
- Shinato, King of a Higher Plane

the rest are pretty much just beatsticks........

as for support aside Manju......... it really depends on what your focus is........

Top 8 - ritual version:

1. Elemental Mistress Doriado
2. Paladin of White Dragon
3. Relinquished
4. Dark Master Zorc
5. Shinato - King of a Higher Plane
6. Reshef the Dark Being
7. Legendary Flame Lord
8. The Masked Beast
(Wait til Magician of Black Chaos comes. That's going to be #1)

If you want Ritual support, have a diverse level of monsters to tribute for ritual summoning so that you are sure of having the exact number of stars for the ritual monster (if you don't get change, why overpay?). Use monsters that work in the Grave or use it for added benefits (Marie the Fallen One, Skull Mark Lady Bug, Spirit of Flames, Aqua Apirit, etc) or steal their monsters to pay for the cost.
Also, to keep in mind, Fulfillment of the Contract is Premature Burial-type recursion, but when the card is destroyed, the ritual mosnter is removed from play. Makes me want to consider a Dimension Fusion if you want to take advantage of that (but that's too slow to pull off and focus your efforts on. Consider it a last resort at best).
Ritual Weapon is not as good as Fusion Weapon since there are alot more monsters that can take advantage of Fusion Weapon, but the ATK boost is nothing to scoff at.
You've already got Manju down, so that's covered.

Now you've got to protect your monsters. More than likely, you'll be tributing many mosnters to summon a ritual monster. You don't want it bottomlessed or torrentialed immediately after, especially if it doesn't have some kind of ignition effect you can activate upon summoning. Royal Decree can cover those traps, or Jinzo. But the reason I'd rather have Royal Decree is because you might be tempted/desperate to summon a ritual monster and may have to Tribute Jinzo. You can't protect too much against spells, but they aren't a problem then anyway (unless Offerings to the Doomed decides to go for the kill).

That's all I can think of. Let's see where you go with this.