ok let's take a closer look at a couple:
mk: fricking impossible to get IN THERE and HIT him with something... but if you get him off the edge it's not TOO hard to edgeguard him (not KO him necessarily, but actually HIT him with attacks), as long as you're read for upb.
snake: his **** priority and knockback on his fast moves is really annoying. luckily, his smashes are slow and easy easy to avoid... and he's really easy to edgeguard (assuming you don't get spiked XD).
zelda: ok, i MAIN zelda, but how come every other zelda i face is a din's fire spammer? *ugh*... still, they don't have any OTHER tactics (besides farore's wind attacking, which is RIDICULOUS when used continuously), so killing them is easy.
olimar: all his attacks surprise me. funny thing though is that sheik's crappy dair actually goes THROUGH his usmash, it seems >_>... so that takes away the annoying.
pit: i have this one annoying pit spammer friend. the trick is ducking the shots. it's funny because he doesn't seem to realize that you can bend the shots down... and when he uses angel ring, i send sheik's chain straight through it for the sweetspot
rob: dsmash dsmash dsmash dsmash.... i was EXTREMELY annoyed with rob until i realized that, rather than ROLLING away from him, it's better just to shield through a dsmash first, then making a break for it. anyway, as long as you're just out of range of his dsmash, he isn't that hard (or at least, the spammer robs aren't hard) or annoying.
i think the REAL annoying is a toon link spammer. you dodge all the projectiles and get TO him... and he does an usmash or dsmash... then he runs away and throws bombs at you... and his aerials are really fast too... ugh... what i do against toon link is just stand all the way across the stage, where he can ONLY hit me with his easily avoided/reflected arrows. when HE does the approaching (rather than me approaching), it's pretty easy... but still ANNOYING as hell.