I will go from least to greatest.
It's only decent stat is Attack at Base 90. It has Low speed and Low bulky. The only way it can be really used is setting up Agility first. Pretty much one of the worst bugs out there, rather sad because of that interesting movepool.
Not much better than Ariados, and even that is debatable. All it really has is high S.Defense. Though if you can pull off a Swords-dance and have Iron-fist you can do some damage with Ice-punch, Thunder-punch, and Drain-punch. He also has Light-screen and Reflect. Aside from that there is not much reason to use him.
About the same as the last two, it has more speed but less Attack than Ariados. You could give Krickitune a Choice-scarf and do some damage, and he has Technician to boost up Bug-bite and Aerial ace (And his Fury cutter starts out stronger if you want to take the huge risk of using it). So yeah pretty much equal to the other two.
Dustox...well it has ok bulk for an NU bug... and it has Quiver-dance, but not really Special-sweeping stats. I think Game-freak was trying to make a wall with this? They definitely failed. The only thing it really has over the first 3 is Quiver-dance, and every other Pokemon with Q-dance uses it better.
OK Beedrill is definitely better than the last 4. He has decent Attack and Speed (for NU) and he has 80 S.Defense which is nice I guess. He has Acrobatics which is awesome, U-turn, X-Scissor, Swords-dance, Brick-break, and Poison Jab for offense, as well as Tailwind and Toxic-spikes for team support. He can be useful in the lower tiers, he is worth a try.
This one can actually use Q-dance as it has 90 S.Attack, but even still it is the second worst Q-dancer in the game. It has Giga-drain for healing, Whirlwind and Tailwind work nicely for some support. But yeah it is pretty much equal to Beedrill.
Butterfree is different, it has the amazing combo of Compound-eyes and Sleep-powder. It has use this to make the enemy unable to do anything while Butterfree sets up anything from Quiver-dance to Tailwind. It can also Paralyze with Stun-spore and blow away with Whirlwind. Definitely useful in the lower tiers.
Leavanny is the bug for Gen V, or at least I assume so because it is the first Bug in the Unova Pokedex. Easily the strongest before stat boosts, 103 Attack stat is quite impressive in comparison to the previously mentioned bugs, as well as a nice 92 Base speed. Access to Swords-dance and has Chlorophyll which can make it a strong sweeper with Sun support. Strong moves like X-Scissor, Shadow-claw, and Leaf-blade. The only thing that it really lacks is Brick-break, this guy is useless against Steel types. He does have Calm-mind so you could run a surprise special set as well.
All in all the best Physical one is Leavanny and the best Special one is Butterfree.