How Do You Pick Units? How Do You Build Your Army?

Old School here and I am hoping to open a discussion about how we choose our lists and the thought that each of us puts into it. Hopefully, we can all learn something pretty beneficial from hearing about how each of us builds.
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So the questions are; "How do you pick your Units?" and "How do you build your army?" These are two questions that are very similar, but there is a difference between the two. The first thing I do is look at the codex, get a feel for the units I really like the idea of both in the "Looks cool" catagory and the "I hope it is that awesome on the table" catagory.
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I take those couple of units and I put them onto paper. I look at their attributes and I pick a theme. I say, "I want to build a high-initiative assault army with multiple deployment options, what can I use to do this and can I support it?" and then I begin the process that ends with an army that matches my goal and has the support it needs to make that idea work.
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In the case of my tyranids, the Trygon model was something I knew I wanted to build, paint and see all over the table ... so I put one down on paper. I then picked genestealers because of all the things I have always loved (and hated) when I see Nids fielded, it is the genestealer - not to be confused with Wynona Rider (the Jeanstealer - lame pun).
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So now, I look at these two units' attributes. The Genestealer and the Trygon both have Move Through Cover and Fleet. One can Infiltrate and Outflank and the other can Deep Strike. One completely eliminates armour saves and the other does sometimes.
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So now, I am looking at units that have speed and can close with the enemy quickly and can deploy in more than one way. With that quick movement, comes lack of Synaps, so I upgrade the Trygon to a Prime and because I want it to get the jump on Marines, I give it adrenal glands.
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Now I have a theme going and I have started to see the answer to my "how" questions. From here, I grab another unit of stealers because two is always better than one and I give the poison sacs. I do this because killing most of a squad due to re-rolls actually helps them stay alive when the enemy replies.
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Now my army is fast and can deploy flexibly and has better initiative than most other units, but that Prime is lonely. I give him a friend. Now I have two Primes, but I know there are hammer units out there that will hand my Trygons their asses due to their lack of invul. I need a support unit - not only that, but one that can keep up with them, so I grab a Hive Tyrant with wings and Paroxysm and a Lash Whip and bone sword. This guy can fly and offer support to either Trygon Prime, even if they get seperated. He can gimp units that are quicker than my Big Bugs with the lash and can make really killy powered units hit like girls with Paroxysm. He is also likely to get ignored with two big nasties in front of him.
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So, now I have my HQ and two troops ... wait, Genestealers are probably going to die ... I need better troops, so I grab the tervigon and one 20 man unit of guants. The 20 guants will easily hold the first objective in my deployment zone and will likely go ignored. The tervigon is decent for the ability to give feel no pain where it is needed and to give me more scoring units that once again, will likely go ignored. My Tyrant also gets Hive commander so I can tweek the deployment style of one of these units to fit the theme of my army. People can feel free to shoot the tervigon as well as she will likey not play a huge role in anything other than sitting around having babies. Every shot at her is one less on my front liners.
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Speaking of the front liners, I now have all my troop choices in place and they are numerous, but still fragile. My army is also starting to feel like an assault spearhead, which it is, but it needs a can opener and what better than two squads of Hive Guard. They are perfect for what I need. They slow down transports that dare approach my deployment zone, enabling my trygons to support. They also open transports and slow them down for the big bugs to hit easily and devour.
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Now all I need is a big distraction unit that fits my theme of my army and it usually come in the form of Doom in a drop pod (everyone wants to kill that guy) or two lictors (another nasty distraction that fits the genestealer-type statline I am going for everywhere).
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Now I have about 210 points left and I am thinking that my assault spearhead could be a little thicker, so I buy a plain trygon and give him Adrenal Glands.
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And that is roughly the process whenever I build an army and when I pick units to fit in that army. So now, I turn the keyboard over to you all. How do you build your army and how do you pick the units. Have a good example. Let's here it!