Ascalam wrote:25mm Fender washers my friends... they will save your sanity
Metal Zoanthropes - always headbutting the enemy.
Stormboyz. Love the models but unweighted the effers will NOT stand up when you deepstrike them in.
Scourges ditto. Gorgeous models, but not prone to balancing.
Eldar Harlequins.
Hormagants (but you can use the claws to prop them up

)
Nightbringer and Deciever were a nightmare in metal. Not so bad in
FC for balance but still a pain.
Old metal gargs. New ones are fine.
the list goes on and on

I imagine old metal Harlequins were a pain, but finecast ones are pretty easy to weigh down as they weigh next to nothing.
I just thought of someone:
the old Metal Sammael on his jetbike. Good god. I've broken more of the clear plastic stands than I care to count utilizing mine, and it's cause his torso and cape to break off the rest of the model in the process more times than I care to remember. I did a decent paint job on it, willing to trade him for the Finecast version!