Easiest is kumquat (Fortunella), and Calamondin (which itself is a Citrus x kumquat hybrid).

KUMQUAT TREES meiwa, nagami, fukushu, sunquat. If you have little use for limes stay away from nagami.

I have been told by many that macrophilia is the rootstock of choice for potted culture

Without a kumquat tree you are not complete.

I would suggest a Ponderosa Lemon. While many suggest Meyer Lemon is easy, I think they are very finiky IMO, but many agree and disagree about this.
Anyway, my Ponderosa was my first citrus ever to fruit for me, in its first year, and the plant itself was not big at all. So it does not take long to mature to flower and fruit. The llemons are also Huge compared to regular lemons, anywhere from the size of a large navel to a small grapefruit!

But if you don't want another lemon, a kumquat is a good choice because they are hardier usually which is good for us northerners. I would also suggest a Kishu Mandarin.
Christy

Kumquat are easy to grow, but not for fruiting if they don't have a lot of heat and sun. In my opinion, a Satsuma mandarine is more grateful.

Kumquats need no more heat than true Citrus to set fruit, but hey are hardier and more insect and disease resistant in my opinion.

I believe that the meiwa kumquat and the indio mandarinquat do well with less heat. The nippon orangequat and nagami kumquat require more heat

I'm not the only one who thinks that:

http://www.hungrymonster.com/food-facts/citrus_guide.php

If you like kumquats, try a meiwa. I grow mine entirely indoors in a sunroom (on Long Island). Easy, easy.

mandarin1

Could you post a picture of your meiwa. I would like to see it and it's setup so that it gets enough light. Do you know what rootstock its on. My citrus are seed grown and still quite small. If you wish to look I have a very short video of the 5 of them.

Steve

Here is a link that might be useful: https://plus.google.com/photos/111099372377958308731/albums/5869826671114565057/5923300816998549858?banner=pwa&pid=5923300816998549858&oid=111099372377958308731

Manderin..Yes, I would love to see a pic?

Do you get lots of fruit and do you peel before you eat!?

That is my next one..By the way, 80's there this week...ENJOY!

I can't say if any of the Kumquats are easy since I have yet to get one, I think Mike is rooting one for me, but I too think Ponderosa a very fun plant to grow never fussy at all!

MIke

This is an interesting discussion because, as some point out with kumquat, it is easy to grow and makes lots of fruit.....but only fruits when it's warm enough. Others recommended the Ponderosa lemon. It does seem to grow like a weed and is very hardy....in certain areas. I don't see how folks in northern states can grow any citrus without a very sunny sunroom but some do quite well. We can share our experiences with certain plants but even in my neighborhood, the soil types vary hugely from clay to sand and a neighbor has a very healthy satsuma orchard but my property probably stays too wet. Bottom line, there are LOTS of variables even if you pick the perfect tree!

I love my Satsuma mandarin. Mine happens to be an Armstrong, but there are many varieties. I am getting ripening fruit right now. My daughter and I fight over the fruit. It's so tasty. Downside, I guess, is it won't bear again until next year. Not a year-round producer.

I move it into the garage in Winter. Gets to about 50 in there at the coldest. Near a southern window.