Aaaaaaand there we go. ANOTHER "Favorite X-type" thread. However as Bug-types are among my absolute favorites, this one I can get behind.
As ever - also list reasons, not just Pokémon. We want intelligent discussion, not SPAM.
In no particular order:
Larvesta and Volcarona
These two became instant favorites of mine the moment I saw them. The design alone would have become a favorite - fuzzy solar caterpillar evolving into a regal, majestic solar seraph moth, six-wings somewhat resembling flower petals with sunspots (if Mega Volcarona is ever a thing it seriously needs solar-flare-inspired traily bits) and some lovely contrasting hues that somehow blend in together into a really lovely color scheme is very much eeee-inducing. The type combination as well as the stats and attacks certainly helped too - and the lore inspired such powerful mental images in my mind that it is not surprising that it became one of my top favorite Pokémon of all times.
Joltik and Galvantula
Anyone who knows me knows that I am immensely fond of spiders and consider them to be some of the most adorable creatures on this planet - and Pokémon was direly lacking in that department. Sure, we had Spinarak and Ariados - which are adorable on their own right but sort of fall into a rather lacklustre early game Bug/Poison category. So when Galvantula was first hinted, then revealed, I was ecstatic. And then Joltik happened and... Yeah. They're made of cute, they kick ass, and they represent a different class of spiders than Spinarak/Ariados (specifically, active-hunter type spiders as opposed to web-lurking types). And of course, they bring the awesome Compound Eyes + Thunder combination into the fray... Yup. Instant faves right there.
Scyther
Certainly my favorite bug of the first generation, Scyther speaks volumes to me - mostly on account of being green and having armblades, but also on account of blending the arthropod, dinosaurian and anthropomorphic together into a lovely mixture. Of course, Technician makes it even better.
You may have noticed I've not mentioned Scizor. It's not that I dislike Sciz - I am actually quite fond of it! - I just don't feel like it particularly belongs as an evolution of Scyther. I feel it should have been a standalone Pokémon, honestly. But I guess they had to show off Item/Trade evos in gen 2 and... that happened. Ah well.
Heracross
Bug/Fighting is a good type combination. A VERY good type combination. And though in Generation 2 it was much less useful, Heracross became an absolute godsend in later gens, particularly in HG/SS (where it made Whitney's Miltank not-a-threat to pretty incredible levels. Also it's pretty damn adorable. Just saying.
Scolipede
Lemme just say that Unova was seriously awesome to bug-types, and hidden abilities make some of them even more amazing. Speed Boost is one hell of an ability, and coupled with a species like Scolipede... Can we get a hell yes? Thought so. All hail the centipede-horse. xD
Leavanny
Another lovely Unova species with a type combination I truly love and some of my favorite moves - even if it is not exactly the most viable. I'd love to see where a mega evolution takes it.
Paras and Parasect
Paras is adorable. Parasect is a glorious fscking tank. They have the same type combination as Leavanny. They're also favorites.
Honestly, there's a lot of species I absolutely love and these are just a set of top favorites . Bug-types are just great, okay? xD