Merritt Hey, can you explain real quickly why Machop, Nincada, Lileep, Castform, Kecleon, Chinchou, Geodude, Seviper, Mawile, and Whismur are in the tiers they're currently in?
Machop - Fantastically strong fighting type that comes relatively early, has what's often a functionally 120 BP fighting STAB in Revenge starting at level 25, no dead period as it immediately comes with the workable Karate Chop, good TM movepool with Strength already available before obtaining it, makes good use of Dig if you want, can use the Rock Tomb, Earthquake, or even the Flamethrower/Fire Blast TMs if you want but certainly doesn't need them. Matchup against Flannery is acceptable - not really a positive or negative overall, although Torkoal is a poor match for Machop/Machoke. Tears apart Norman, beats up the rival for the most part, sits on the sideline for Winona and Tate&Liza, beats 2/3 of Maxie/Archie's team, can usually take on Sootopolis even if it doesn't sweep entirely, beats Sidney and Glacia like they owe money, Phoebe's a no unless you give EQ in which case it can usually take on one or two (keep in mind it always has guts), mediocre vs Drake but usually can take out a Flygon at least, 4/6 of Steven's team lose to the champ, and you can probably take Armaldo if you don't try to take on everything else as well, Wallace is less friendly but can usually take out a mon or two - especially with EQ.
Nincada - Fireburn summed Shedinja up well enough, go read his post and the following few if you still want an explanation. It's not F at the moment because it's extraordinarily good at taking out the things it can beat. Ninjask doesn't deserve F but considering that its movepool until after goddamn Winona is Secret Power and Dig (neither of which are STAB and are both low power) it's not contributing much if at all. Aerial Ace is still pretty weak - 90 base attack isn't spectacular and 60 BP is definitely not, and Shadow Ball is even weaker and is moderately high demand. Baton Pass allows Ninjask to finally achieve some real use, but at Level 45 it's probably not going to be a factor until E4.
Lileep - Asked the guy who convinced me to put it there in the first place.
It's stupid to grind up but it doesn't die against pretty much anything without se stab. you throw it on your team and any holes you have against bosses is covered if you slap toxic on lileep and let them die. giga drain and sludge bomb only speed up the process once you hand them to lileep. it might not be a world beater but it's definitely not f rank trash lol
Castform - Currently at D because its TM movepool allows for great coverage that's let down by poor stats. Weather Ball's alright at best - sure it's Castform's signature gimmick but a lost turn every time you do a battle adds up quick, especially when you can use a different move instead. Might be worth moving down to E, it's definitely on the worse side of D.
Kecleon - While its level up movepool isn't exactly amazing (Slash is alright but Strength is better), its TM movepool allows it to at least contribute against whatever major battle you want. While Color Change is annoying, it's not exactly debilitating since it usually leaves Kecleon resistant to whatever the opponent's strongest attack is, and Kecleon has barely enough Attack to punch with the endgame opponents by abusing its TM movepool. Bulk is pretty good too, helping offset the dead speed.
Chinchou - Chinchou's flaw is that it comes very late. Otherwise its movepool is fantastic (STAB thunderbolt and surf is excellent, toss it Ice Beam if you want but it doesn't really need it), its stats, while HP heavy, are alright because of its high BP moves, and it ends up with a good matchup against the remainder of the game. It's like a slightly later, slightly worse Staryu.
Geodude - trade or not-trade? They're somewhat different stories, although the common point is busting through Wattson-Winona (Graveler less so against Winona overall unfortunately), falling off somewhat lategame, and Golem making stronger contributions against Tate&Liza and parts of the E4 compared to Graveler.
I'll edit in the others later because
holy shit that's every Pokemon mentioned on this page I haven't already explicitly addressed.
e: Alright let's deal with the others.
Seviper - Speed isn't that bad - still outpacing most route trainers and a lot of major battles' slower mons. Movepool works well as a mixed attacker - the special Bite/Crunch and Flamethrower pair well with Sludge Bomb and Earthquake if you choose to use it, allowing Seviper to put in work in pretty much every major battle in the game, Tate&Liza being pretty much the only hard no and even there Seviper can help out if needed. It's fairly comparable to the also D rank Grimer, having roughly the same level of power on Sludge Bomb and more special attacking power with a little more speed in exchange for lessened bulk.
Mawile - Emerald Mawile is garbage, being quite literally the last Pokemon available before postgame. Ruby Mawile manages F purely on stats alone, being extremely good for that point in the game, and not falling off for a decent while later. Its movepool is unimpressive but suffices for helping against rival 2, somewhat against Wattson and Norman, and can be used a little against Winona although you're better off slaving it at that point, as Intimidate makes it at least somewhat useful for support in battles unlike most pure HM slaves. Essentially Mawile's a decent crutch to get you through earlygame into midgame, and so doesn't reach the levels of awful for F tier.
Whismur - Whismur is a slow, frail, and fairly weak Pokemon who evolves moderately early into a less frail, still slow, still kind of weak Pokemon who doesn't evolve until lategame. Whismur chugs TMs like steroids in a frantic attempt to remain relevant, since the only thing it really has going for it is that, but virtually all those TMs that make it usable come in the mid-to-lategame where Loudred is gasping along, frantically struggling to contribute by constantly spamming a moderately effective Strength against everything when it knows its stats began to fall off at Flannery. After shoving a metric ton of CDs into its gaping maw, Loudred pretends that everything's ok but deep inside it knows that it's really not and that it's still frail and slow. Then it finally manages to drag itself into evolution and it's alright but the game's close to over and Whismur is really really terrible holy shit.
e2: It occurs to me that perhaps Whismur isn't quite as dispassionate as it should be. Maybe I'll edit it later. Basically, go actually try out Whismur sumwun.