Togepi
Get killed in Purity Forest and wish cave most of the time from its Metronome (Dragon Rage, Blast Burn, Sonic Boom, Sheer Cold, etc), so I usually have to use my strongest attacks to kill it b4 it does anything really bad.
Kecleon
I can steal from them with no problem, and easily pwn them (I have my Meganium eat so many powerups, it is practically a powerhouse), so getting away with it, but they never join me, and b/c I spend so much time trying to recruit one, I am always getting killed by a gust of wind.
Golbat
Screech+Leech Life= Instant KO at like 200+ damage.