I usually let the terrain and my allies' choices decide. I feel picking different facs than your allies makes it inherently harder for the enemy to fight you.
Cloakbots and spiders are my current favorites. Cloakbots on somewhat hilly terrain, spiders on very hilly terrain.
I'm currently on the fence on whenever I prefer rovers or tanks for flatlands. Tanks feel stronger overall but feel like they can easily be swarmed and out-maneuvered early on. Whereas rovers are great at doing said swarming and out-maneuvering, but it feels like the window of opportunity where they shine closes quickly, at least on lobster pots where player density is high (which is pretty much all that I play).
I like to play gunships but am not actually good at it, so I usually reserve them for when no one else is going gunships out of a large team, and I have a nice backspot to start in.
I don't like the jumpbot's factory lack of reliable generalists so I rarely play it as a first fac, but sometimes do as a second.
On the sea, amphibots are my favorites, while ships are an essential secondary factory if the enemy isn't a pushover. Hovers sometime happen, mostly if the sea itself is not valuable (no mexes) and I'm only using it as a stepping stone for land raids, or if other people have started as the other facs.
I never strider rush because that is crazy and bad and you should feel bad about rushing striders. I sometimes build them as a second or third fac, but usually wait so long to do so that the game is basically already decided by the time I do. I have been trying to get into the habit of making them earlier, when things are starting to get really dense.
The two facs I never play are shieldbots and air. Shieldbots because I don't like relying on shield balls, air because it requires a ton of micro to be effective.