Hey there! :D. I think I can give you a hand with it :3. Even if Exeggutor is horrible for the Elite Four, I think you should still have it on your team honestly. I'm saying this because most Pokemon are useless for certain situations ;;, but really great otherwise! And Exeggutor is one of those Pokemon for me, it has a great moveset and can be really handy, so I advise you keep it, but it's your choice of course :D.
However, depending on what sort of Pokemon you can like, you can pick out of either the two. If you like speedy, but less bulky Pokemon, I think Exeggutor would be a great choice. And if you love defensive bulky Pokemon, keeping aware of its speed, I think Slowking could also be useful, although it might take a really long time to raise for the Elite 4 and having two Pokemon of the same type on your team isn't what I really like to be honest. :(
But it's your choice! :D. I hope this advice was useful and if you still have trouble choosing, feel free to ask again. :3