Full Dragonites are cool. Full FP Venusaur is useful bc it has two bars. Smack Down Tyranitar is top rock attacker. If you can push for a high level on any of those three they would be good to trade for. Realize you will lose some IV values unless you wait for friend level to significantly build, so level and immediate usefulness is better than %IV.
GamePress
Community Day move Pokemon trading value
Which of the community day Pokemon are actually worth trading something rare for?
Also, which is more important to trade for, considering you can only get one (because you only have one shiny that someone wants to trade you for): Legendary Pokemon like Kyorgre, who just left, or exclusive move Community Day mons?