OK so I just beat the 7th gym and my team stands at Smuglord / Fossil Turtle / Centipede / FireGorilla / Mole / HMSlave. My thoughts on each of the following-
Smuglord- The only thing this guy really has going for it is bulk and speed. It doesnt get good coverage moves; I have to deal with Leaf Blade / Return along with Coil and Giga Drain for my moveset. Smugleaf does decently well during early game though, Grass Mixer is a pretty damn awesome move, especially along with an early Growth. In fact, you can just Growth up on anything that wont do super effective damage to you, and then just sweep the remainder of the opponents team. Its evolutions are also pretty good at tanking through tough fights. But IMO Wotter would be a better starter considering the lack of very good water mons in this game, as well as it having better offenses (75 / 75 for Smuglord is very mediocre) and better movepool in Megahorn and Revenge by Heart Scale and level up respectively.
Turtlemon- I got this as soon as I could, and seriously, this guy is a fucking beast. he comes at Lvl 25 when you revive it- to put it in perspective, Gym 4's strongest mon is at Lvl 26. Sure you have to trek back to Gym Town 2 to revive it, but its worthwhile IMO. Solid Rock and Sturdy are both great abilities, and it gets Aqua Jet early on to bypass its abysmal speed. You can then teach it Rock Tomb via TM and Dig / Smooth Over (from gym 5) to complete its moveset. Crunch is pretty helpful as well. Its evolved form kicks even more ass- it learns Shell Break at lvl 40, Aqua Tail at 45 and Rock Slide at lvl 51. Im struggling to say how good this turtle dude is, its just sooo goood.
Centipede- You get the first form at around lvl 15-18 in the forest after 2nd gym, and it learns some good moves soon- Poison Tail at Lv 19 and Bug Bite at Lv 22. But after that, its a pretty big burden until it evolves into the final form at Lv 30. And then it starts to shine. 112 Base speed is excellent, and 90 Attack is good enough ingame, especially with access to high power moves such as STAB Megahorn from Heart Scale. It can also learn Smooth Over and X Scissor, TMs which you get soon enough after evolving it.
FireGorilla - You get its prevo at lvl 15-19 in the desert, even higher in the hidden part, and it comes with Fire Fang and Headbutt. If you leave the building of trainers in Hiun city for this guy, you will get your powerhouse in no time. I got mine at lvl 15, took it through that building and came out with a lvl 19~ powerhouse. The only trainers it had some trouble with were the Basurao users. The Bug gym helped it level up till 22, when it learns Fire Punch. The gym leader was just OHKOd throughout, apart from the Rock / Bug who should be no trouble for any other mon not weak to Rock Blast. After that it just keeps getting better, with Flare Blitz and Thrash fairly early on. After evolution, encourage Flare Blitz just OHKOs everything. Thrash and Hammer arm also give good coverage alongside. This mon singlehandedly makes the Fire monkey and Fire starter absolutely redundant. A must have on every team.
Mole- I got lucky with him and managed to catch a lvl 10 prevo in the first cave in a shaking patch. Its a bitch to raise until lvl 19 since its so frail and you cant really take foes out with Fury Swipes and Metal Claw quick enough, but after Dig at lvl 19 it starts to get better. Slash, Claw Sharpen and Rock Slide all in the 20s is great, and evolution at 31 is pretty decent too. However, one may be better off holding evolution till level 33 when the prevo learns EQ (as opposed to lvl 36). After this he gets Swords Dance at lvl 42 and by then you get a moveset of SD / EQ / RockSlide and one of Slash / Xscissor / Shadow Claw, which is pretty awesome considering your core moves are all learned by level up.
I also used Terrier until gym 5, and it was a pretty damn good tank. Intimidate was helpful, and STAB Take Down just OHKOd everything, and Crunch was great for hitting ghosts. But then it starts getting redundant and you get tempted by other, more exciting Pokemon on your journey and just have to let the little guy go :(