I am planning a OTK deck involving dumping tons of cards into the grave thru Reasoning and Monster Gate. However, I need to decide on which monsters to put in.

DMoC is an auto-in, but what other good targets would there be? (Preferably not one of Lv4 or 8 since those would be commonly called by the opponent for Reasoning)

Well, Gilasaurus is helpful when you want to use Monster Gate. Special Summon him, then tribute for Monster Gate. Also, Sangan might be good. Besides, what FTK deck do you have in mind?

It would help to be more specific on what sort of OTK you're trying to do....otherwise, there's no way for people to know what you're looking for...

Unless you're playing Traditional and trying to use Reversed Worlds/"Exchange of the Spirit"...

If you are playing traditional (I really hope not), go for Magical Scientist FTK.

If you want an example, go search through my threads, and look for the thread FTK at the very last page.

Another fun advanced format OTK is Magical Explosion. Just have a small amount of monsters, and reasoning everything into the graveyard, and activate Magical Explosion for the game.

It is very, very fun.

"Exchange of the Spirit" you just use Reasoning to dump 15+ cards (hopefully you get Spirit to your hand first though, in Traditional) and use it during your opponent's Draw Phase.