Ghasts for me are passable in the solo campaign, simply because I love watching all those M2 peasants being terrorized by them. In multiplayer they're a bit odd imo.
bre'Unor are funky - I just wish they were a little more accurate.
Myrks irritate me when they start going berserk on each other, although it is an interesting twist to gameplay.
Have to agree the Myrk Giant is my least favorite M2 combat unit overall (if I had one to remove). I definitely prefer the Forest Giant (I love that WHACK sound and resulting physics). Myrk Giants are so annoying when there is only one left and they can just dodge/throw forever - even more annoying than Trow in this respect. I believe they are tossing "exploding skulls" - not sure where they come from though. Their death sequence is cool but not entirely well explained (to me it hints they are some magical version of Myrks).
Another on my list is Skrael - I think only because of their color - I'm not sure of the whole TFL history of these guys (I never played that far) but I guess I would expect them to "blend in" a river/water environment more.
But I think the overall award for me has to go to the M2 Peasants. They are
insanely annoying. It was a great pleasure of mine to convert them to other stuff throughout all my plugins. BTW, if you ever want to
really enjoy killing them, you should play StoneHeart RDF Hunting on one of the "outside" maps with extra tough units (try the Rumble maps for Forest Giants or do Marathon AW) - why? - because I made a special
MiniPeasant(tm) with everything (including sounds) scaled and there are 100 in the StoneHeart to hunt down and keel like rats (yep, they even scurry like rats). It's creepy to hunt them with Spiders/Ghols/Wasps yet strangely satisfying. I occasionally play one of these maps solo and see if I can kill all 100 in under 6 or 7 minutes (it's tough because you can't auto-target them). I've been thinking of making a separate solo game of this (Peasant Hunt) just to formalize the whole thing. Of course, using RDF Rocket Dorfs or RD Giants is particularly satisfying...
MiniPeasants(tm) await their fate on SHRDF Rumble...