I began my first playthrough in episode 10, So I couldn't experience the field effects in all their glory. In my recent one I already know what to expect, so it doesn't really count.
Hardest - basically those who took more then two times to beat
Shade: At this point in the game his team is really horrifying, and lack of any dark type in my team didn't help. Beated him on third try with one pokemon left.
Hail Team Serra: This was the moment I realised I can't keep my HM slave with me and must have a full team. But because I wanted to know what will happend next in the story I just took a level 15 Vulpix as Drought support. Still hard battle though.
Honorary mention to Aya who was able to give me really hard type in my mono-steel run despite the disadvantage
Easiest - defeated on first try
Radomus, Luna, Samson