Kumquat and Calamondins are probably the vey easiest for containers, indoors. Calamondins themselves are a kumquat x mandarin hybrid; it does have the advantage of very aromatic flowers (I don't find kumquat flowers particularly aromatic, even though I do love kumquats).

My calamondin is by far the most tolerant tree I have. It is an easy beginner tree. I havent had a fruit yet (it started off as a baby stick) but it is supposed to be sour. (I love sour food!) Whatever you choose, just make sure it has a lot of light, is fed well, and is in the right mix. That is what I have surmised to be the key to growing citrus.

I agree with subtropix and Amanda, calamondin and kumquats are very easy to grow... and they are quite forgiving

I love my new calamondin, I have tried kumquats on many occasions and find them incredibly bitter/inedible. I used to cook for a living and many fancy pants restaurants used them and I always dreaded taste-testing those dishes. But on the flip side many people love them. I think they are both easy to grow and prolific fruit producers.

poih