Ropers: because even the stalagmites want to kill you.
10 HD, which are fortunately Magical Beast rather than Aberration type. Large size, and pretty good stat boosts, all above-average. However, I'd expect a bit more strength of a creature that's going to take up half your build.
Ropers interact in seemingly arbitrary ways with the elements, like all good D&D monsters do. Cold they resist, electricity doesn't hurt them at all, and for some reason they're vulnerable to fire. They also have SR 30, which while not quite impossible to overcome for mid-level creatures sure isn't easy either. Then again, remember that SR is a double-sided sword, and it'd sure be great if that Heal spell you desperately need actually worked.
In addition to their natural bite attack (Mouthpick ahoy!), ropers have six strands. They're these oddball ranged natural attacks, which don't deal damage but grab people. Interestingly, the strands don't actually seem to work that well RAW: all one does is drag people closer 10 feet each turn, but it doesn't seem to stop them from walking away again, or even moving outside of the 50 ft. range. I guess one could attach multiple strands, though.
Weakness somewhat solves that issue, though. I think it's supposed to deal 2d8 strength damage to a creature the moment you grab it (though it may be an activated action, or a continuous effect), which would mean that if nothing else the six strands per round are a decent debuff, mostly just burdened by the fortitude save involved.
Ropers are a strange monster. They're only decent at their supposed main job of dragging close and biting them, but depending on the way you read Weakness they've got the potential for mid-range elimination of spellcasters and other weak targets and debuffing of enemy bruisers. The low move speed is incredibly annoying, though. I suggest getting a dozen Talismans of the Disk, or something like that.
I think +0 LA will suffice here. Discuss!