I'll answer this now that I've played through the game, although I won't give six Pokemon, but multiple choices and explain why, along with advantages and disadvantages of starters.
Serperior: Easily the worst choice for ingame, seeing how Su-Seeding isn't good for ingame. It has an incredibly poor movepool, and poor attacking stats. Dragon Tail is also needed to fill out a Physical moveset, and it isn't really the most useful move for ingame, especially since you get it so late. You'll be spending most of the game with Grass and Normal type attacks. Also, Vine Whip will make things harder for you early on, due to such a low base power.
Emboar: Tied to be the best ingame starter. It has a huge Attack and HP stat, and can even use Special Attacks well towards the start of the game. As long as you keep it at a high level, that low speed stat won't become noticable until after the E4. Ember helps you very early on, and it soon gets Flame Charge, which is a powerful attack early in the game, and helps out with Speed. As a Pignite, it gets Arm Thrust, which not only provides good type coverage, but also deals with the multitude of Sturdy Pokemon in the game. Rollout also proves nice while you wait for Rock Slide. It also gets a powerful STAb attack with Heat Crash. Assurance helps in Double Battles you fight in, especially along with your other useful moves like Rock Slide or Earthquake. Type coverage combined with incredible attack make this a powerful Pokemon that an destroy foes with neutral hits. He tears through the Gym Leaders with ease also, having a Super-Effective attack for most of them.
Samurott: It can learn Surf. YAY. This guy is also good for ingame. It gets Water Gun early on, and later Shell Blade, which has really high base power at that part of the game. You can also give it Retaliate after beating Lenora. It gets plenty of coverage options that work well with stats that are pretty much perfect for ingame, like X-Scissor, Megahorn, Ice Beam, Aerial Ace, and Grass Knot.
The Simis: They work great for taking out the first Gym Leader, learn plenty of HMs, get good attacks with plenty of coverage, can evolve pretty early (Castelia City if you want), and you get it for free. They also have excellent sweeping stats. Simipour is likely the best for ingame, closely followed by Simisage, with Simisear in last.
Watchog: It gets STAb Tackle and Bite at level 6, Crunch at level 16. If you keep it around for a while, you can also give it plenty of type coverage from TMs, it being the Normal type it is. The most useful function for it though is in catching Pokemon with Hypnosis, Super Fang, and Mean Look (along with Illuminate for an ability).
Stoutland: It starts as a Lillipup with the very helpful Pickup ability. STAb Tackle and Bite at level 8. Good ingame stats, plenty of type coverage from TM or the fangs, and we all know how much damage Take Down can do from Lenora. It also gets Surf!
Liepard: You get it early on, so you can get it to a high level pretty quickly. Mostly, just use it to bolster your team in the early part of the game, and maybe keep it with you if you get it to a high enough level. Fake Out is useful for Sturdy Pokemon.
Musharna: You get Munna around the start of the game, and can soon hit the foe with STAB Psybeam. A healing move at level 17, and a Sleep move at 19. Synchronize is a useful ability if you have the right nature. It gets pretty good coverage from TMs, and has good stats when you evolve it, making it hard to take down and allows it to hit pretty hard.
Unfezant: You get it early on, so use it to bolster your team at the start. It can also use Fly. Quick Attack is pretty useful.
Zebstrika: You get it pretty early in the game. While you won't get a STAB move till level 11, you get one that is pretty good, especially since the line has good Special Attack along with high Attack. These guys are incredibly fast, even more so after getting Flame Charge at level 15. At level 53, it gets Thrash, which is an awesome move ingame.
Excadrill: Okay, those stats are great, and Steel typing is still powerful ingame. STAB EARTHQUAKE!!! Yeah, that can tear through any foe, especially coming off of that high Attack stat. It also gets enough coverage from TMs. A bit rare, but it makes a valuable addition to any team.
Gurdurr/Conkeldurr: Depending on whether or not you're able to trade. Even if you aren't it is still powerful with a base 105 Attack. And base 80 Attack is excellent for a first stage Pokemon. It gets Low Kick and Rock Throw early on, offering great type coverage. Later these can be replaced by even more powerful attacks. While not the best TM pool, you can let it be your Strength user.
Seismitoad: Incredibly balanced stats, great for ingame. The typing is also great, leaving it with only one weakness. It gets STAb Surf, and STAB Eartquake, which are two of the most useful moves in the game. And even early on, it does good damage with Bubblebeam and Mud Shot. Uproar is a failry good move ingame, and it can later be replaced by the excellent Hyper Voice. Drain Punch is also great as it keeps your health high. It has plenty of useful type coverage from TMs. One of my favorites for ingame.
Leavanny: Nice attacking stats for ingame. X-Scissor and Leaf Bladeare obtained pretty early on, and before then it gets a powerful Bug Bite and Razor Leaf. It can also learn False Swipe to help you catch Pokemon.
Sigilyph: This guy is amazing, getting excellent type coverage, able to use Fly, great stats, Magic Guard, hell, this guy is just amazing, especially since you can get it before the 4th Gym Leader.Use it and you will not be dissapointed.
Carracosta: One of the best HM Slaves in the game, Dive, Surf, Waterfall, Strength. Yeah, this guy is great for the job.
Gothitelle: Frisk is a nice ability so you can take items from Wild Pokemon with Thief. It has nice stats, and a good movepool. It gets Psybeam and Psyshock early on, so it can do good damage with those.
Swanna: HM slave.
Galvantula: Okay, a fast Pokemon wth nice Special Attack running around with STAB Bug Buzz and accurate STAB Thunder ingame is something to be scared of. While TM type coverage isn't the best, all you really need is Thunder and Bug Buzz, then just give it Energy Ball for Rocks/Grounds and Thunder Wave for catching Pokemon. Electro Web is a nice move for before Thunder, it does decent damage and lowers the foe's Speed for sure, and attacks both Pokemon in Double Battles, so lowers both of their Speed. You should also be getting Electro Ball and Signal Beam soon after catching it. You can also evolve it fairly soon after catching it, especially since you'll now have the Lucky Egg to help you out.