I'm not sure what to teach my Venusaur, I know Leech Seed does the same amount of damage on nearly all pokemon but Giga Drain would do more damage (especially when my Venusaur has a modest nature so it has higher Sp.atk).
You could try both! If you're playing defensively you could go with Leech Seed and Giga Drain to keep your HP topped up, and hold Leftovers. Poison your opponent with Toxic and you're home free (maybe).
Venusaur doesn't have a great attacking movepool, the only other special move you'd use is Sludge Bomb which is mainly neutral coverage. Perhaps Earthquake for better type coverage?
The times I've used leech seed with my skiploom, the damage\HP absorbtion doesn't seem to be all that much. The advantage of leech seed is that you don't have to reuse it every time unlike the "drains". If you want a good amount of damage\HP absorbtion each time, I go with giga drain. This is according to FireRed so might be a bit off since yours is Diamond.
It all depends on what your looking for. If you want a fast move that regain helath and can be used every turn, leech seed. But, if you want something that has alot of power, but, can be only used every second turn, then Giga drain. Hope I helped!
Well if you have a hier defence you should probably but both
because then you will be undefeatable and if you perhaps
want to teach him ingrain and give him leftovers to hold that should be great.
but if you dontwant that you should probably teach giga drain.
all of this depends on your pokemon other stats.