Please, take in account that this is just my opinion... I'll list from Best to "worst", based on my experience with Reborn
Well, Okay:
Torchic is a given... Blaziken is too OP, and there's a very good reason as to why it's Ubers nowadays... Blaziken will destroy everything in it's path... And what it doesn't destroy, no worries: You have 5 other pokemons, so whatever... Blaziken IS the best choice. I don't even care about type advantages or wekaness in this case, Blaziken is power... Just don't use it against Chandelure...
Froakie is probably the second best, but it ties with the next one (Hang on)... Since Greninja gets Protean, and the A.I 99% of time is not going to predict the type change, it can makes some battles real easy... So, if you don't like the KFC chicken, i'd say Froakie... Protean is OP = Greninja is OP
Piplup ties with Froakie IMO, for one reason: Steel... There's not a lot of steel types that you can get in the start of the game (Klink? Eeh?) and... Well, Piplup is not a steel type, but Empoleon is, and Empoleon is good enough... Even with the nerf in the 6th gen, Steel is very good... So Empoleon is very useful... Piplup and Prinplup might struggle a bit on the start, but it will get better... I guarantee it...
Chimchar... I love Chimchar... It's my starter, and honestly: It made me win a lot of gym battles in one try... Iron Fist is good, and remember: If Blaziken doesn't have Speed Boost, Infernape is superior... I'd say Chimchar is the third best, but the synergy in my team helped a lot, so idk... I recommend it, but maybe you should think twice about it...
Charmander... Well, you know... No one likes 4x times weakness... BUT: How many trainers have you seen using Stealth Rock in Reborn? I've seen none (Actually, Shelly uses Stealth Rock. But you're not going to have a Charizard there, so whatever. Thanks Sky ^^) ... Charizard is good IG... And remember: Mega evolutions in the future, so that Charizard X/Y might be yours in the future... Also, Charmeleon gets Dragon Rage, the most OP move in the start of the game... So, yes... Charmander is also good...
I have no opinions on the other starters... I have on Turtwig, but you didn't add it, so i'm not going to say anything... Just two things:
1 - If none of these starter that i listed "appeal" to you, choose Treecko... Between all the ones you're going to choose, Treecko is the last option... IMO
2 - All starters struggle against the first gym, believe it... Even if Marshtomp can do good, so what? The 2nd gym is Grass, and it's goign to f him up...
The most important thing is the synergy between your team... The starter is really... "just the start"...
So yeah, that's what i wanted to say... For reference, i voted on Torchic ^^
Good luck o/