I always choose based on personality type (as in getting all personality types in my town), colour palette and whether I like their overall character design or not. With colour palette in particular, I'm hoping to create an autumn-themed town soon, so as such I'm looking for villagers that are either brown, green or orange.
Normally when choosing my dreamies, I look through the personality categories on nookapedia and choose that way. I always have one of each personality, so it does make it a little easier. Often I either want two peppies, or two smugs since there's only 8 personalities. Then I choose my dreamies based on who i've loved having in previous towns, as in villagers who've become my favourite villagers as they're a must have in all of my towns. After deciding that, I start picking other villagers to go in my towns based on if I think they'd look cool and if they'd go with my other dreamies colour scheme wise. Usually before searching for them on the forums I tend to go on tumblr to look at some screenshots of them before deciding if I definitely want them or not, because they could look cute in pictures, but actually not so cute in screenshots.
For my main town that's Pokemon themed, the dreamies I picked are all based on Pokemon themselves. It was a difficult process to pick only 10 but I wanted to make sure they all had the following requirements:
1. The villager must somewhat look like the Pokemon I'm aiming for them or at least close in species.
2. 8 of them would belong to one of the four human characters in my town (2 villagers/Pokemon for each trainer/human, including the mayor). Because of this set-up, the final 2 were very difficult to choose because I still wanted them to follow the Pokemon theme.
3. Their greeting and catchphrase were to based off of a Pokemon attack/theme and species of Pokemon the villager is representing. This was the most difficult aspect to maintain because the game makes it so easy for them to steal each others' catchphrases. It's a shame their greeting doesn't transfer through for dream addresses. ;^;
Their houses was the thing I looked at the least in terms of choosing. Originally, I was going to decorate each of their houses but it's so time consuming and it's a shame you can't change their wallpaper/flooring. I'm still decorating their houses little by little or restoring them to their original state (way easier said than done).
As for my second town, I was going to do a historical/medieval theme but I found myself getting uninterested after getting dreamies I thought I wanted. Now it's slowly starting to become a free range town with no theme to make my life easier. I heavily based my decision on their design and houses before going free range.
So yeah, it's been quite... the process haha.
I just googled a complete list of New Leaf villagers and picked the 10 I liked best :3 I have all of them now thanks to awesome tbt traders.
I just realized i replied to someone else who had copied your post :/
but!
I looove Pokemon, I'm so curious to know which villagers you picked to match them!