Parasect with Fury Cutter/Solar Beam

Parasects can always be useful for training imo. I personally don't transfer any of them unless they got Cross Poison, and I use them very often against Eggs (the ones with X-Scissor) and Vapes (the ones with Solar Beam).

Keep it for prestiging grasses - I've got a dozen Parasects (Paras is very common round here).

900 CP? Sounds like you don't have maxed slots if you're only thinking about keeping one. In which case I would def transfer. Paras is mad common so not like you can't evolve another.

Parasect is a prestige Pokemon only. Maxed out, it just doesn't have the stats to compete.

90% of his prestige value is vs. Exeggutor. Other 10% is Vaporeon/Water types (w/ SB as charge)

Since he's got SB, you're looking at Vape/Water Mons. In order to find a Vape in the 1700-1800 CP range it would have to be ~ lvl 20.

Not sure about you, but I don't see many lvl 20 Vapes in gyms by me.

If in your area you see 1800ish CP Kingler/Golduck as defenders that's about the only time it'd be useful.