For the first of B-, I have Magnemite. Magnemite's sky high special attack, Steel Typing, and Electric STAB allow Magnemite to act as a good offensive threat. Magnemite can utilize its Steel typing to wall Pokemon, most notably Abra, and Choice Band Doduo, letting Magnemite set up a Substitute or throw out an attack. Magnemite has some flexible moveslots after it puts its attacking moves, meaning it can make room for utility such as Toxic, Substitute, or Metal Sound. Magnemite's high special attack stat means that very little things like to switch in or take its moves unless they resist or are immune to them. Despite these good qualities, Magnemite is very hard to pilot and needs to be played carefully to get value on a team due to Magnemite's terrible matchups. Magnemite gets trapped by Diglett, and can't OHKO Trapinch, meaning it's also trapped by it, Magnemite loses to offensive Wailmers when they press Earthquake or Water Spout when healthy, but a slight remedy for this issue is that Magnemite can outspeed and OHKO bulky Wailmers, Magnemite loses to Doduos pressing Hidden Power Ground, loses to Snubbull with Earthquake, loses to Anorith and Bagon with Brick Break, loses to Ponyta, and probably some more. Magnemite still walls a select few Pokemon and can provide value to a team when used right, but keep in mind that is difficult to pull off.
Next I have Larvitar. Larvitar is basically a worse Bagon. It serves the same niche as Bagon as a Dragon Dance sweeper, and Larvitar has a few things over Bagon, but a few things under it. Larvitar is a stronger Pokemon, even though Bagon has higher physical attack, Larvitar has STAB moves that give it the offensive edge over Bagon. Larvitar has access to Guts, giving it a better Duskull matchup, as it can't threaten Larvitar with Will-o-Wisp. Despite these good qualities, Larvitar is much worse than Bagon due to its typing. While STAB is nice, the weaknesses that come with the Ground/Rock typing are very important. There are many good Water type attackers in the meta such as Wailmer, Staryu, and Chinchou which all threaten a non-boosted Larvitar, the quadruple Grass weakness makes Larvitar susceptible to surprise Hidden Power Grasses from Pokemon like Elekid and Ponyta, and the Ground weakness makes it weak to Trapinch and Diglett. Larvitar is indeed a good offensive Pokemon, but its terrible defensive profile makes it very hard to use.
Now I have Cubone. Cubone has the highest attack of all viable mons with Thick Club. Almost nothing wants to switch in on Cubone's incredibly powerful attacks, mainly Bonemerang, which is an incredibly powerful STAB move considering the strength of multi-hit moves in LC, Hidden Power Ghost which can hit Gastly and Duskull, Double Edge which Cubone can use without recoil due to its Rock Head ability, and Rock Slide which can hit Pokemon like Doduo. Cubone also has a high physical defense stat, meaning it can endure more hits from physical attackers like Diglett, meaning Cubone has more chances to attack. Cubone's explosive power does not come without its flaws however. Cubone is incredibly slow, meaning it is usually going second, and taking a hit. This problem is exacerbated on Cubone due to it having to run Thick Club to be viable. This takes up its item slot, meaning it can't run Sitrus Berry, and therefore can't sustain itself, so all the damage it takes is permanent. Cubone has impactful weaknesses, such as Water, Grass, (look to Larvitar why those are important), and Ice, which means Elekid can hit Cubone super effectively with Ice Punch. Cubone is also very specially frail. Cubone is also countered by Choice Band Doduo. Choice Band Doduo can switch in on Bonemerangs safely, then OHKO Cubone with Double Edge. All these flaws make Cubone incredibly hard to use well, but it still is possible for Cubone to provide good value to a team.
For the last of B-, I have Bellsprout. Bellsprout serves a niche as a sun sweeper. Exeggcute is generally better, but Bellsprout has the unique trait of being able to OHKO Houndour with Sludge Bomb (Granted Bellsprout needs attack investment and a Poison Barb), a Pokemon which walls Exeggcute. Bellsprout is able to outspeed the entire metagame with Chlorophyll active with minimal speed investment, and proceed to throw out very strong Solar Beams and sun boosted Hidden Power Fires. Bellsprout is walled by more things than Exeggcute, is easier to play around, and doesn't know Explosion, making Bellsprout generally worse than Exeggcute. Bellsprout is walled by Gastly, and needs Spikes support to be able to OHKO Pokemon like Ponyta and Anorith consistently. Bellsprout is also very frail, meaning it is difficult to set up sun consistently. Bellsprout can indeed put in work in a game, but the conditions needed to do so are difficult to achieve.