Given that you are greenhouse growing, I would recommend dwarf varieties like Holiday.

1--- http://www.tropicalfruitnursery.com/avocado-viewer/index.htm
2--- For openers get a Hass. Those are the familiar ones from California used in guacamole. Hass is oily and greasy and full tasting but not all avocados are. They can actually be watery and bland
3--- Believe it or not some avocados are more cold resistant
4--- Dwarf variety? Any mango can be pruned small and perhaps the same with avocados

Here is a link that might be useful: avocado cultivars

little cado is supposed to be a dwarf variety, i prefer Mexican hass over the Caribbean varieties as well

I have only tasted a few avo other than the normal CA ones (Mexicola, Hass) but at the Festival of Fruit a few years ago I tasted a Sharwill avo, it was incredible. We also had some softball sized avos in Puerto Rico that were excellent, dont know the variety though. A friend has been collecting lots cultivars of avos, when they all start producing it is going to be a tasters paradise.

-Ethan

Thanks for those.

Yeah, I'll definitely need to keep them pruned pretty tightly to keep them off the ceiling. So more than size, I'm interested in flavor.

I have become quite acquainted with the Pine Island cultivar viewer, and I appreciate their descriptions, but they're in the biz of selling avocados, so I'd prefer to hear it from avocado lovers directly. Knowwhatimean?

check out these 2 videos about avocadoes from Dave Wilson nurseries. It is very informative. I have a Haas and a LittleCado. I do not have fruits on them yet, but these are the 2 varieties highly recommended to me by people in my area.

http://www.youtube.com/watch?v=7z5ONrV67MY
http://www.youtube.com/watch?v=tXpH5Kpzxy8

Those are great videos. Super informative for me. Thanks Ashley!

I also recommend sharwill as a delicious avocado; worlds better than the commercial 'haas' variety overgrown in CA. Reed is another really good tasting avocado, round with light green, smooth skin. For a big avocado with very rich taste, there is Butterpear. Good luck.

http://www.youtube.com/user/DaveWilsonTrees

Dave Wilson is very good with lots of educational youtube videos. His nursery is in Modesto California. His basic theme is to get a lot of different fruit trees, plant close together, prune and prune. Or get say 3-4 different apple trees. Whole idea is to get fruit for many months. Keep many smaller fruit trees instead of a few big ones. He gets to sell more trees too :)

Here is a link that might be useful: Dave Wilson- high density fruit trees for your back yard