Ike? He hasn't given me much trouble online, a few can use him to annoying degrees but I manage to work with it.
Something I find odd is that I virtually never see anyone use the allegedly "top-tier" Brawl character online, that being Meta Knight. Personally, I suck with him to extraordinarily shameful levels (I'm talking THREE Easy Mode deaths on All-Star), but that's hardly unusual since the tier rankings have pretty much meant jack [parasitic bomb] to me from day one. Besides Marth, anyway.
The guy (using Mario) gets a star, and runs over me. Normally, it's not a big deal; after all, it's what most people do when they get one. However, the guy switches into reverse and runs me over again. Then he pulls forward again and hits me again. The annoying part was that it was inescapable. He hits me with this "combo" of his until the effect of the star runs out, and then he pulls off, despite the fact that he was already lapped. The other opponent took advantage of this and won that race.

In "Mario's" defense there is a legitimate reason for practicing such tactics: Snaking revenge. Thankfully most of them have moved on, but if you are unfortunate enough to run into them again, annoyance is your only recourse (well, besides hacking a Battle stage and waiting for them to do something that crashes their game, thus forcing a disconnect/loss on their end). I myself would turn my kart around and race backwards. The decent players would take a hint after that. The assholes paid a visit to Pipe Plaza.

I don't play a lot of online games so I don't have too many bad online experiences. Besides temperamental nights with the Smash Bros. servers, anyway. >.> The worst of those would probably be when they kept on kicking us out of my "Dragon's Lair" stage while we were trying to have a 2.0 Damage Coin match. I don't really understand that at all, maybe the Nintendo servers just couldn't cope with the sheer number of flying character ricochets.
Then there's Pokemon Battle Revolution. Some guy came at me with a Spore Breloom and Fake Out Mew. Took me three tries to dismantle it, and when I finally had victory in my grasp, the connection was conveniently lost.