What armies best represent the lower tier of power level?

I gave my votes to (in order from worst to less worst):

Bretonnia - the lack of an updated codex hurts them a lot, especially if you consider that 8th isn't really an edition that loves cavalry charges
Beastmen - if I want a brutal maneater army, Ogre Kingdom is much better
Tomb Kings and Wood Elves: they both can give you good results, but it's like to play FHFB - Hard Mode. You cannot afford a single mistake.

Edit: just a clarification about WE.
I don't have a real experience with them, i've only faced WE twice, one playing lizardmen and one playing dwarfs, and those were amongst my easiest victory in 8th.
Almost everybody tells they're strong, so clearly they require a high level of competence to perform adequately.

I'm surprised you said Ogres and Dwarfs - Ogres are one of the few armies I actively struggle to beat, while Dwarfs I would argue are more powerful than some others I could mention (I am a Dwarf fan so I may be biased, but who cares), indeed I would argue Dwarfs are far better than Empire for example. Dwarf infantry is miles better, their artillery can be given runes to make them more reliable and their characters not only have better base stats but can also be tooled up better with their runic items. People may say Griffons and Demigryph Knights, but even they won't last long to a concerted barrage from Organ Guns and more reliable Cannons.

That's fair - from what I've heard Beastmen can still be a powerful force if used extremely tactically in situations like that.

I'm not sure I agree that Wood Elves are in the low tier. I think they get a bad reputation because they are very difficult to play, but if you play them correctly they are very powerful. That makes them a poor army as an out-of-box experience and makes them susceptible to complete collapse in battles, but not in my opinion a low-tier-power army. I think the mark of the weakest armies is the armies that even when played well will struggle to come out on top of an event. Tomb Kings, Beastmen, and Bretonnia are obvious candidates for this in my opinion. They can definitely win battles and can win a lot of battles if played well, but I think that they will really struggle to beat opponents that are just as good at playing and are bringing powerful lists out of other books. I don't think Wood Elves share this weakness. A well played and well designed Wood Elf army can be extremely powerful and difficult to beat. I'd rank them in the upper level of mid-tier.

But not better than WoC!  hahaha       (just bugging you!)

The Herdstone is pretty cool.  

I agree with you that Wood Elves are definitely not bottom tier (at least in my opinion).  I too would rank them around the upper-mid tier.  In the hands of a truly exceptional general, they might even creep into the top tier (as you said, they are a finesse army).  Plus they cause a lot of problems against some of the best armies in the game (High Elves and Dark Elves).