Talking from a strictly stats point of view.....
Grass
Meganium
HP:80
AT:82
DF:100
SPAT:83
SPDF:100
SP:80
Total:525
Weakness:5
Resistance:4
Immunities:0
Normal:8
Most Powerful Move: Solarbeam Lv:61
Fire
Typhlosion
HP:78
AT:84
DF:78
SPAT:109
SPDF:85
SP:100
Total:534
Weaknesses:3
Resistance:5
Immunities:0
Normal:9
Most Powerful Move: Flamethrower Lv:60
Water
Feraligatr
HP:85
AT:105
DF:100
SPAT:79
SPDF:83
SP:78
Total:530
Weaknesses:2
Resistance:4
Immunities:0
Normal:11
Most Powerful Move: Hydro Pump Lv:55
According to the stats, Cyndaquil -> Typhlosion is the better of the three to choose, but then again, that depends on how you plan to use the pokemon.