In no particular order, fifty way to annoy:

Severus Snape

1 Call him Snivellus

"Hello, I'm James. What's your name?" Snape looked up at the boy standing in the doorway of his train carriage. He was a first year, like Snape himself, and he had a mop of untidy hair that stood up in all directions.

"My name is Severus." A snort of laughter came from behind James, who stepped aside to reveal another boy, who looked to be the same age as them. He was taller, and had short, slightly ruffled dark hair.

"Sorry for not introducing myself earlier. I'm Sirius." The boy said formally. "Nice to meet you, Snivellus."

"My name is Severus." Snape informed him frostily.

"Yeah, well. Mind if we sit here? No? Good." The two sat down opposite Snape. After a while, Sirius glanced over at him, and muttered something to James, who grinned. He then turned to address Snape.

"Want to see some magic, Snivellus?"

"My name is Severus, and no. We're not supposed to do magic outside of school. It's forbidden."

"Not on the train, surely? Anyway, it's not as if anyone will care. Here, watch this." He lifted his wand, and flicked it sharply.

"Orchideous!" He announced, and a bunch of orchids sprung from the end of his wand. Admittedly they were a bit pale, and one or two looked slightly see-through, but on the whole, it was rather impressive. Sirius sat back, with a smug smile on his face. "I'd like to see you try something like that, Snivellus."