I like the shambling mound. Mind you, not as a monster; as a monster it's pretty terrible. No, I like it because it's a perfect example of the weirdness of WotC's monsters. "Yeah, we made the plant resistant to fire, immune to lightning, allowed it to swim, then slapped a LA on it because our guidelines for what should be a PC are completely arbitrary."
Mechanically, mounds have 8 plant HD and some reasonable stats (assuming you go melee). They get two slam attacks (which, given that they're giant vines, can probably not be used to wield weapons with), slow land and swim speeds, Large size with reach, natural armor (which is good because just try to find magic armor your size), improved grab and constrict (kinda relevant at these levels), with the aforementioned fire resistance (not how plants work, guys) and electricity immunity.
Speaking of said immunity, shambling mounds have the ability to gain points of constitution any time they'd be damaged by lightning. If you read this sentence and thought 'can it get to infinite?' then congratulations; you're a typical member of the GitP forums!
The answer is yes, by the way. One point is lost every hour, so as long as you get shocked a dozen times per day or so you'll slowly gain more and more con. And of course, with as little as three class levels one can get a source of at-will lightning damage. Guess why this is getting an asterisk?
With no infinite constitution shenanigans, shambling mounds are simply plants with decent strength, two natural attacks, and enough HD to be useless. -0* LA.