what is considered the bottom house?

The honest truth is that all the houses here are great. I know, so cliche, and I sound like a broken record. But it's said so much because it's true. If a house here is not popular, financially secure, and supported by strong nationals then it won't succeed here, i.e. why Sigma Delta Tau is as small as it is and why other sororities have colonized and failed completely. That being said, houses like APhi, DG, GPhi, Theta, DZ, and AOPi are all extremely strong in their own ways, though it may be portrayed differently on this site.
These houses tend to get placed in the "bottom" tier because one, every house needs a place when you rank and two, when you do rank, these are the houses that kind of get the short end of the stick. I personally hate when people rank houses because they rank all the Old Row houses top. No hate to Old Row whatsoever, but if you honestly take away Old Row status, you'll see that a handful of New Row houses work just as hard, if not harder, than most Old Row houses. For example, ADPi, DG, and Theta are all extremely philanthropic and involved, a lot more so than KD, AGD, and DDD. ADPi and Theta also have extremely high GPAs, whereas a few of Old Row houses have some of the lowest.
Overall, none of the houses you listed are "bottom" because they're "bad." They're ranked lower because not every house can be top.

Yea you pretty much got it, a lot of people also consider gphi and dg bottom tier but above those. Amazing girls are in all houses though, not to sound cliche. There's nothing wrong with ending up in any of these. All houses at bama will give you a great experience.

Don't listen to anything on this site. Everyone's out to get each other and to put houses that cross rush down to make themselves seem better. Its a privilege to get a bid from any house at Alabama and with that attitude you should be thankful if you do. Go into every house with an open mind and look for girls that you click with.

Bump- this post is so accurate and a really important thing for PNMs (and actives tbh) to understand