Best sunny defender

Probably Venusaur with Frenzy Plant.

It resists fighting moves, which makes it go well in a gym with the typical defenders.

Then it has a multi-bar huge charge move to do damage on its own.

for sunny weather you want fire and grass type defenders, possibly with a poison, flying, or psychic subtyping. so venusaur with razor leaf/frenzy plant, charizard with fire spin/blast burn, exeggutor with bullet seed (or confusion/extrasensory/zen headbutt)/seed bomb/psychic, a high cp sceptile with leaf blade can also do a lot of dmg on a sunny day, vileplume and victreebell would be some lower tier decent sunny defenders as well

The best defenders that would benefit from sunny weather I think would be RL/FP Venusaur and C/SeB Exeggutor.
Venusaur is a good defender anyway, resists Machamp and sunny makes FP even more punishing.
Exeggutor is likewise a good Machamp wall but there's a huge difference in performance between Confusion and other fast moves. Bullet Seed is a horrible defensive move even when weather boosted. Alolan variant with DT/SeB is also ok but doesn't stop Machamp.

If you're really scrounging for more defenders, FS/FT Flareon does hit back very hard in sunny weather. Unfortunately it's not very bulky and most trainers have rock Tyranitars now to hard counter fire types.