The hardest thing about takedowns is that, just like any other technique, you can't just "know them" and hope to apply them. The standing game is its own separate entity from the ground game and it requires knowledge of how it works as well as many hours of practice to get good at. To apply any takedown, you need to be aware of not only your own positioning and balance, but also your opponent's position, posture, where his weight is going, and how he's going to react. A really good book on this is "The Judo Advantage" by Steve Scott. Your takedowns will also change if they're wearing a gi, if they're much stronger... there's a lot to think about.
With that being said, the "easier" takedowns are ones that require few moving parts and more control of the opponent's body overall. Double legs are a good example of this, as are things like body lock takedowns or duck unders. Throws and foot sweeps, while still great takedowns, are much more difficult to get if you're not super proficient at them. They are difficult to set up and you have to have a good sense of timing for them, as well as good technique.
Anyway, here are some you can look at:
No gi: Slideby/throwby/Shuck, ankle pick, snapdown, and armdrag to ko-uchi-gari.
Gi: Collar drag, tomoe nage, ankle pick