Exeggutor or Venusaur against groudon?

eggy

Yes.

If you have sunny weather, Venusaur and Exeggutor are consistently in the top attackers on PokeBattler's raid rankings.

Exeggutor has higher attack, but is hindered by weak Bullet Seed or other fast moves that lack type advantage.

Venusaur is better if you can dodge. And there are preliminary reports on Reddit of people claiming that the dodge bug might possibly finally be fix with the update that is currently rolling out.

Eggy resists SB but afraid FB. Venus double resists SB but afraid FB and Earthquake. I vote for Eggy.

Eggy has higher DPS so generally Eggy. If you value tankiness, a sufficiently high level Venusaur can live through 2 Solar Beams from Groudon, which Eggy will never be able to do.

All great analysis!

My Exeggutors get destroyed with the death/dodge glitch (often without even getting a charge move off) so it's Flying types or nothing for me. I've been going Gyarados, Articuno, and Dragonite exclusively since I realized the pattern.

Both

Gamepress analysis is kind of confusing...Gyarados and Dragonite are good against any moveset, even Solarbeam.

If Groudon has FB Vaporeon gets a spot as a good counter.
For SoB Exeggutor and Venasaur are interesting even though Eggy can't survive 2 SoB (just as Gyarados), but offers high dps.
For EQ you can mix your Favorite grass and water mons.
Victrebeel is completely useless in any matchup! Ho-oh is an interesting choice in any matchup if it has SoB and you lack counters.