I run with Bug mono-types any time I play games from Gen I and II, as a way to give myself a challenge. Gen I there's only 6 fully evolved Bugs anyway, so I won't bother with a list there. In Gen II, I typically ran with...
Of course I'd have to go with my Dragons but if those weren't an option, it'd be either Fire or Dark, my other 2 favorite types.
My Teams are pokemon that i like and enjoy with moves i feel would be great combos with and also sometimes help their teammates out, so nothing perfect but something i could work with
Kingdra (Sniper) - Dragon Pulse, Surf, Ice Beam, and Either Octazooka or smokescreen
Dragalge (Adaptability) - Dragon Pulse, Play Rough, Sludge Bomb & Water Pulse
Tyrantrum (Rock Head) - Headsmash, Iron Head, Outrage & Crunch
Flygon - Sandstorm, Earthquake, Quick Attack & Dragon Claw
Shiny Hydregion - Dragon Pulse, Dark Pulse, Flash Cannon & Flamethrower
Mega Charizard X - Dragon Rush, Flamethrower, Thunderpunch & Steel Wing
shiny Ninetales (Drought) - Flamethrower, Energy Ball, Extrasensory & Confuse Ray
Houndoom (Early Bird) - Flamethrower, Dark Pulse, Iron Tail, Thunder Fang
Microwave Rotom - Overheat, Thunderbolt, Shadow Ball, Thunderwave
Infernape (Iron Fist) - Fire Punch, Mach Punch, Thunderpunch & Stone Edge
Arcanine (Intimidate) - Close Combat, Morning Sun, Extreme Speed, Flare Blitz
Mega Charizard Y - Solarbeam, Flamethrower, Focus Blast, Air Slash
Dark: (One of my favorites to mess with, soo many combos to be had)
Honchkrow (Super Luck) - Drill Peck, Night Slash, Aerial Ace, with either Haze or Quash
Spiritomb (Infiltrator) - Dark Pulse, Shadow Sneak, Destiny Bond (takes out any pesky powerhouses), Ominous Wind
Shiny Greninja (Protean) - Aerial Ace, Rock Slide, Night Slash, Water Pulse
Drapion (Sniper) - Cross Poison, Night Slash, Toxic Spikes, Ice Fang
Weavile (Pressure) - Ice Punch, Night Slash, Aerial Ace, Low Sweep
Mega Tyranitar - Crunch, Rock Slide, Iron Head, Earthquake
I apologize for necrobumping this, but I've crafted Ground, Dark, Fighting, Ghost and Poison-type teams that I keep in battle boxes; however, my pride and joy is my Bug Monotype team. Bug: Scizor (offensive Defogger), Vespiquen (Special wall/Toxic staller), Golisopod (Emergency Exit momentum keeper), Volcarona (Quiver Dance set-up sweeper), Galvantula (Sticky Web setter), Scolipede (Speed Boost speed control)
I've found mono-electric to be pretty good in monotype battles. My team consisted of Alolan Golem, Tapu Koko, Magnezone, Zapdos, Rotom-Wash, and Xurkitree. If I were making a team of favorites I'd go with psychic and pick Starmie, Claydol, Lunatone, Solrock, Gardevoir, and Sigilyph.
That would be a tie between Ice and Fire. I know both are on the fragile side, but both pack huge punches, and Fire does have the second most resistances of all the types (Steel, obviously, is first). Plus both are my favorite types, especially the poor, neglected Ice type, as I love the cold and snow. Now... let's see...
[In Progress] Mono Steel (Pearl): Bronzong, Empoleon, Bastiodon, Steelix, Riolu/Lucario, [Blank spot] (Not enough Steel types in the game other than Trash Wormadam, which means fumbling around with honey mechanics. Maybe Skarmory will take this slot in the post-game.)
A while back I made a monotype team for each type on Pokemon Showdown for a fun little project. Given that this was using the Smogon Monotype format, I did use quite a few legendary 'Mons in most of my teams, but here are a couple of my favorites:
This was a great rain team and was my favorite to use, and it sort of built itself once I decided to use rain, with Pelipper as the setter and Swampert and Kingdra as the abusers. Protean Greninja was the next obvious pick because Protean in a monotype team is kinda cheating TBH but I'm gonna roll with it. Lanturn was a must for its Volt Absorb ability, providing a valuable secondary electric immunity alongside Swampert. Toxapex is there because 'Pex.
This was my other favorite team, and I built it because I absolutely love Aurora Veil. Alolan Ninetales is the best user of the move, and the extra bulk made it super easy to set up with either of Clefable or Azumarill. Tapu Bulu is there because he breaks things in half, while bulky Togekiss does a nice job at taking hits while fishing for flinches with Air Slash. Mega Diancie is used to set stealth rocks while offering some form of hazard control with Magic Bounce.
Sableye and Mimikyu were mandatory picks to offer resistances to dark types. Jellicent and Sableye form a nice defensive core, both having great bulk and access to Recover. Alowak and Golurk form a nice breaker duo, the former being able to wield a Thick Club to get the equivalent of a free Swords Dance, while the latter equips a choice band alongside Iron Fist to maximize the amount of things it can punch apart. Gengar holds a choice scarf for speed control, while Mimikyu abuses its Disguise to set up a Swords Dance in order to clean up.
Mega Charizard Y
While this one contains the second-most legendary 'Mons of all my teams (my Psychic team ended up with legendaries in five of the six slots), this was my favorite team to use. Given the flying type's weakness to rock-type moves and, by extension, stealth rock, this team was built around supporting Mantine to make sure it could keep hazards off the field. Zapdos runs a SubRoost set to take advantage of its defensive typing and just generally be annoying, while the rest of the team aims to pressure the opponent offensively as much as possible.
This team surprised me the most with how effective it turned out to be in spite of all of bug's weaknesses, but by taking advantage of bug's offensive prowess, it turned into a rather fearsome offensive squad. Galvantula sets sticky webs for the team and aims to get a kill or two in the opening exchanges. Forretress runs rapid spin for hazard removal because defog is kind of counterproductive to sticky webs offense while also being able to stack stealth rocks on top of Galvantula's webs.. Araquanid provides a valuable fire resistance and wears an assault vest to turn it into a formidable special tank, dishing out massive water-bubble-boosted Liquidations. The last 3 'Mons are your pick of setup sweepers, with Scizor and Pinsir both offering priority as an additional form of speed control.
If I had to choose one of the above, I'd pick either my Water or Bug team. I love my Water team because I love dedicated Rain teams, and I enjoy my Bug team because it showcases the offensive prowess of a type that is generally considered among the weakest.
Water is the one I like the most in practice, lots of options and all the types available as secondary. Although Steel is probably the one that would fit me the most as a person, and it has some great options too.
I could actually make up a monotype team of Electric-types with the Pokemon plushies I've collected. I own more plushies of Electric-types than any other type. The Electric-type also suits my personality as I'm always on the Internet and I'm a wannabe game dev (of visual novels in particular).