It's hard to evaluate their in-game strength (in-game, since we're talking about their non-fully evolved forms too) because they're all so different.
- First we have the general basic and stage I (TCG references ftw) with the Cherubi family. They evolve at a certain point in the game. Level 25 to be exact, which means about halfway through. When they evolve, moves become slower to learn, but they become much more powerful. So it is generally a good idea to evolve it straight away.
- Next we have the basic stone evolution with the Sunkern family. Stone evolutions will usually stop the Pokemon from learning any new moves, but not with Sunkern. For Sunkern the moves are merely different. And they're not too bad either, which is good, since Sunkern is one of the weakest Pokemon with only 180 base total base stats, at 30 for each stat. That means that if you don't EV train it, it will likely be at level 100 with double digit stats (shock, horror).
- Next we have an even weirder evolution scheme with the Roselia family. From a baby that requires incense to be bred, evolving via happiness to a basic and then again via a Stone. Not only is the Budew to Roselia evolution unpredictable (unless you actively attempt to evolve it), at its Budew form it learns very few moves, branching fron Roselia from level 13 and stopping to learn any more moves at level 16. When I'm not actively trying to evolve them, I've had Pokemon evolve via happiness at very low levels and at the same time I've had them evolve at level 40 (Golbat to Crobat in Silver). Net you have t decide when you want to give up on learning those level up moves and evolve it to Roserade with a Shiny Stone. Once you have, it will cease to learn any more level up moves.
- Our last family, the Oddish family is also very unusal. We start with a normal level evolution at level 21. Next we have to decide to evolve it to either Vileplume or Bellossom, possibly swaded by which stone we have. For Bellossom we lose a type and a tiny amount (10) of Special Attack in exchange for Special Defence. The dropping of the Poison type makes us lose our Psychic weakness in echange for a Poison and Bug weakness. We lose Pedal Dance for Magical Leaf, a weaker move that we might not even learn, since it is learnt at such a low level, 23, just two levels after we evolve to Gloom. On the plus we, we gain Leaf Storm for SolarBeam and take a Ground resistance (a common type) in exchange for our Fighting resistance.
So, I think I'm swaying towards the Oddish family, but that may be because I tend to sway more to older Pokemon, I don't know why, nostalgia probably.