The experimental type — low raw stats, high gimmick ceiling.
Bug has a reputation for being weak, and in the early game that reputation is earned — Caterpie and Weedle exist to teach new players how evolution works. But late-evolution Bugs include some of the most competitively viable mons in the game: Volcarona, Scizor, Heracross and Galvantula all see top-tier play. Bug is the gimmick type, and when the gimmick works it wins tournaments.
Bug is an offensive speed type. Many Bugs run Tinted Lens (Venomoth) or Technician (Scizor) to turn weak STAB into threats. Bug Buzz, U-turn and Megahorn are the three staple attacks that define the type's moveset.
U-turn is the best pivot move in the game alongside Volt Switch. Bug Buzz (90 BP special) is the special nuke. Megahorn hits 120 BP but misses at 85% accuracy. First Impression is a priority move that only works on the user's first turn out.
Scizor's Mega Evolution was so popular in Gen 6 OU (Smogon's standard competitive tier) that it was one of only two Megas banned to Ubers, alongside Mega Kangaskhan.