The best u can be Comfey - RegalZ

Guide

Edit: I edited some stuff since comfey got buffed.

Before anything the items you choose can vary, but no matter what I recommend keeping the sp attack specs since it buffs comfey's healing by a really good amount. For emblems use green and black ones. Now onto the guide.

At the start don't use synthesis too often since it's healing isn't too great. You should instead focus on basic attacking and using vine whip. The only times synthesis should be used is to heal others to save berries, help others escape, or to attach to an ally and de attach ahead of them for more distance.

Once you reach level 4 get floral healing. Sweet kiss is very situational and is only really good for attaching to brawling mons like garchomp, scizor, etc and even then comfeys healing is more valuable. Unlike synthesis, floral healing is worth using mid fight since it heals a lot more. Also when you de attach you automatically get a free boosted attack. Here's two tips to take full advantage of floral healing.

This is a combo for floral healing. When you attach to an ally heal them, de attach immediately, use your boosted attack to get 2 flowers, move into a bush to get 4 flowers, and then when floral is off cooldown attach to your teammate again and heal them. You should have all 8 flowers thanks to your passive. Since floral healings cooldown is pretty small you can do this consistently. It's the most effective way to heal with comfey.

Another thing you can do with floral healing is attach to an ally and keep spamming floral healing. Doing this gives them a nice shield effect if they're at full hp. However there's some requirements for this to work well. You should only do this before a fight happens and you need to stay attached to your ally until the enemy breaks the shield. If you de attach before the shield is gone it will weaken a lot. After doing this strategy just go back to doing the healing combo like normal.

Once you reach level 6 I recommend you choose magical leaf. Grass knot isn't as good as magical leaf when it comes to damage and stun. After magical leafs buffs it now targets enemies instead of wild mons and stuns more. It's range is increased dramatically if you're attached to an allly and also automatically attacks hidden enemies. Floral healing and magical leaf have lots of chase potential if they're used when attached to an ally.

Finally for the unite move. The stealth it gives isn't great, but it's good for defense and sustaining nearby allies. You can use it for defense at your goal zone if you and an ally are outnumbered allowing more time for backup to come or you could use it in team fights to keep your front line pokemon alive allowing them more time to gain an advantage. You also get 8 super upgraded flowers when using your unite move. Meaning floral healing does even more healing than normal. You should heal first before using the unite move for maximum value.