Played through reborn three times and I'm very salty about that Typhlosion ranking lol. Ran a sun team with him and he was by far the best starter I chose. His early game movepool is pretty trash but Nature power can compensate for it until you get Lava Plume and Eruption. You can grab a cherubi before you've even hit your first badge and teach it sunny day at level 22. Set up sun and watch as Typhlosion makes the complete mid game and double battles an absolute walk in the park.
He should be higher than Serperior imo, who honestly couldn't pull his weight on my team until the blessed leaf storm.
Not sure about venasaur as a starter but i've used him on the Typhlosion team and he is pretty beastly after a growth boost.