Given the question and the options given, I'd say that my answer to the question of how I decide what army I want would be… Orks?
My methodology is theme first, if I like the theme of the army, and can come up with a force that I want, then that's what I get, competitiveness comes later. For example, I collect Sisters because I love the models and the background of the army, I play Order of Our Martyred Lady because I really like the colour scheme as it's pretty much what I would have chosen anyway, but I opted for the mourning colours (black cloth with white trim, red lining on the reverse, rather than all red). Sisters are easily the least competitive army, even with significant allies they're pretty weak, but they're fine for casual games.
Dark Angels were my original army when I started
40k ages ago, so I've had a load of them for a while. But I got back into them when Dark Vengeance came out, the unique terminators revitalising my love for the Deathwing, with me focusing on a mostly Deathwing strike force (with some scouts and Ravenwing I can use when I want to), though I have a ton of Greenwing that badly needs stripped and repainted. It's not particularly competitive, but works well when it works, and they're all terminators, which are awesome, the Deathwing models are especially nice (plus I have some Cataphractii as a command squad, pictured really badly in my gallery).
I also have some Eldar, but focused on Wratih elements; currently just a unit of Wraithguard, a unit of Wraithblades and a Wraithlord, plus somehow I've ended up with four Spiritseers (one of whom is a Bonesinger). I also have two units of Warp Spiders that I've converted from Dire Avengers. Oh and a Viper, because I like those. So again, not even a remotely competitive force, but I like the idea of a Wraith-reliant force, but don't like the Iyanden colour-scheme, so thematically I'm thinking my little force may actually be from a Craftworld that is retreating (possibly even leaving the galaxy altogether), but sends wraith-forces through the web way to fulfil outstanding oaths before they are out of range entirely.
I also have ordered the Renegade set so I can get an Imperial Knight cheaply by splitting with someone else, probably going to build as a custom freeblade.
So yeah, these are my most recent armies (Imperial Knight most recently of course), and the reasons are largely "I feel like"