In our game, we mostly ignored the Familarity Requirement - the Druid/Shifter I play began play at 14th level, and it is nearly impossible to say what he already might have seen - Oh, and consider that a character at that level might have access to spells like Legend Lore, Plane Shift, and many more (via his allies), so he could already have seen any creature out there.
Then, remember the Nature Sense ability of the Druid - you could interprete it as him being familiar with any kind of creature, because he can identify it at glance - at least animals (including Dire and Legendary ones) and plants.
Anyway, even without these notes, the following creatures are most frequently used:
Troll (Regeneration is fine, though the Strength is lower than of other, similar sized giants)
Fire Giant (the wizard uses his polymorph self for this creature)
Both are some of the most favorite monsters in use, I think, and nearly everybody will know some of them.
Considering "Nature Sense", the following forms might also be very typical for a Druid/Shifter to be familiar with:
Treant (a very strong creature, terrible combined with Greater Magic Fang)
Tendricolous (oh, a large Dragon? Nice breakfast! - well, in the actual situation, I wasn`t able to swallow the Dragon, because his attack bonus was far to high - but I swallowed a Gargoyle - Regeneration 10 is great, too)
Most other forms I used were more for fun than actual power, like
Aboleth (underwater adventures, though Dire Shark or Whale Forms might be equal or better...)
Delver (How to open doors, walls and anything else in your way - nice for undermountain/underdark campaigns)
Will O´ Was
Forms I use(d) for powerplaying:
Stone Golem (Be aware that most other "Standard" Golems do have the Berserk Ability, which is extraordinary - and without a master, there is little chance that anyone can heal you of it. But Magic Immunity is awesome)
Legendary Ape (might or might not be available thanks Nature Sense)
12-Headed Hydra (no special form, since I consider them as "unaverage"...)
Forms I did not try, but might do (and some of them are even regularly available to Druids)
Bullette - just for beeing a landshark

Dire Tiger
Elephant
Anyway, I try to use as many various forms as I can - mainly for the fun it does. (Shifter are the "Walkin One Man Zoo"s)
If it is getting hard and enemies are to powerful, I will probably use the primary forms like Fire Giant or Golem.
Mustrum Ridcully