Lucario in BW2. He did well early game, but UGH in both my Black 2 and White 2 he could never take a hit. Seriously, anything that wasn't bug or normal just completely wrecked him. Not to mention, in my Black 2 version he was speed gimped.
On a lesser note, Aggron and Steelix. Steelix is a strong contender for my favorite pokemon, but sheesh are these things suprisingly fragile. Awesome defense but still need Sturdy to survive ground/fighting attacks, and Sp. defense so low that I need Sturdy just to survive almost any special based attack. In-game (W2 team) both were completely outclassed by my Excadrill and Probopass (B2 team) who could either outspeed and KO enemies/tank hits for my team and spread paralysis respectively.
Also on Soul Silver Beedrill. Yeah I know I shouldn't have expected much out of him, and he managed to stay useful until the 4th badge (pursuit was a godsend) but he simply got outclassed by stronger, faster, and bulkier pokemon.
Empoleon until I got Surf and Ice Beam. I was used to Infernape who has Mach Punch, Flame Wheel, and Close Combat by the time he evolves. Empoleon only had Bubblebeam, Metal Claw, Peck, and Aqua Jet. Given that he was Modest (and Empoleon is Special anyway), he was quite weak.
I'm not easily impressed; I'm usually oblivious to whatever's in front of me. Pokemon White 2 FC: 3139-7420-3142 - THIEF
I've tried and tried, but I cannot seem to get a decent Steelix. Yeah, it can take a bunch of hits, but it doesn't really learn much. It learns one STAB move, Iron Tail, on its own. No EQ or anything. If it stays an Onix, it gets Sand Tomb...
Unless I'm doing something wrong, I can't seem to get a good Steelix. Which sucks because it's one of my favorite designs, and two of my favorite types.
"Gary was here. Ash is a loser!" = Greatest Tramp Stamp Ever Green & Blue Bird, this is Mr. Fabulous, we are a go for cheesin' it. - Bender