Best: Zos had just introduced monster set motifs you could farm on vet, it was White gold tower. I joined with my tank on a vet run, when I got in I noticed that the group was at the last boss and thinking nothing about it since a lot of people leave as soon as the group wipes once, I pulled the last boss and I noticed that the group was less than optimal to say the least. They died to the lightning walls and I died to the atronach one shot mechanic, then one in the group wrote a sad message "sorry for wasting your time "my name", but we are all new and you won't be able to kill it with us". I thought this was very strange thinking about one wipe, especially since the flame atronach boss is harder than the last boss and they had killed that one and I asked them some questions, apparently they had gone through 2 tanks who had been extremely toxic towards them, calling them a bunch of things and still they hadn't left, because they wanted to kill the boss. So feeling bad for them about meeting 2 super toxic people, when all they had done was being new to the game, I instead of taking the que of the former tanks I just said "don't worry, I'll stay until we kill it, it's really not that hard". So I explained the 2 mechanics you need to think about on voice chat, we wiped once more and tried it again and killed it without a single person dying. The other in the group cheered on voice chat and me as well, congratulating them on a job well done. I said my good bye and left, then I got a long whisper from one of them thanking me, I in turn invited them all to a friends guild and after a few months these new players had completed vet MoL, vet HoF and were progressing on hm Vas and they are still my friends to this day.
Lesson of the day? Don't be a toxic [snip] cursing at people when all you have to do is write one sentence and explain the mechanics instead of bashing on new players.
I paraphrased quite a lot since this happened several years ago and I don't remember exactly what they said.

Worst: I think this is more fun than bad, but it's also about a toxic tank. I was leveling up a stamblade for PvP and I joined a rnd, it was Direfrost keep. I joined as a healer, I was in stam spec, I put on the heal that sacrifices your health to heal a party member, this being a normal very easy dungeon I knew this would be enough. The tank in the group had 810cp with 40k+ health, we went to the first boss and killed it easily, then the tank wrote "another fake healer great..." and I responded confused "I am healing?". He responded with "no you're not, you don't have a resto staff equipped". I just sighed and said to him I don't need a resto staff to heal, he bitched on and proudly said how he had finished every hm and vet trial as main tank and knew everything, I ignored him and we moved on. On the second boss same thing happened, we killed it extremely easy and I healed whenever I needed. Tank typed "why don't you heal me?", confused since I had healed him several times , I said "I healed you several times". He started a vote kick towards me and typed to the rest of the group "we need an acutal healer because otherwise we will never finish this dungeon". I was confused and asked if he was serious. Then I was kicked, he whispered me "that's what you get for not healing, you should learn from your betters".
This was not long after summerset and I answered him with a link to my Z’Maja Shadow Skin and wrote "good luck getting this if you think you can't heal without a resto staff in normal direfrost" and put him on ignore. And I just sat for like 10 minutes flabbergasted about this dudes attitude and didn't know what to think, he was easily the most arrogant person I've ever encountered in eso, especially after seeing how bad he was at tanking, obviously lying about all his end game trials.

[edited for profanity bypass]

Best: Just getting LoM trifecta on a regular run through with group.

Worst: Doing pre-nerf LoM with low DPS groups.