Giant and Evil Outsiders are no-brainers. Usually, I get giants at level 5 (because thats when ogres and trolls start to show up a lot) and EO at 15. The other 3 options are more open.
Undead, Elemental, and Construct are popular choices because all mobs under those keywords are immune to crits, so any extra damage is welcome. Undeads are really common. Even if the quests isn't undead-only (necropolis and such), there are many quests around where a few undeads show up here and there. So, it's a solid choice at level 1.
Monstrous humanoid is a solid choices at level 1 too, and can keep up until 12-13 (Assault on Stormreach and Gianthold). Not really useful after that, unless you run a lot of epic Red Fens.
Reptilian is an odd case. On harbor and market (level 2-5) there are plenty of kobolds and trogs around, but at those levels, no mob is really hard to kill in one or two swings anyway. But you can get those levels where AC is easy to achieve an untouchable level, and get favored defense against them. Then, retrain off.
Aberrations are the last good choice on my list. There are plenty of aberrations after level 10 on game, and almost as common as undeads (not many aberration-heavy quests pre-U9, but many aberrations here and there). But since they are crittable mobs, plain DPS can take care of them easily. I bet post-U10 their rank may go up a bit.
Personally, my choices on a pure ranger are:
15- Evil Outsider
20- Construct or Aberration