+1 vote for this forum's namesake, I really enjoyed the Orange Islands arc. It was cool to see Pokemon competing in a way other than battling with the crazy gyms and Ash's battle against Drake was pretty good imo. The series also has one of my all time favourite episodes; Bye Bye Psyduck, which made Golduck one of my favourite Pokemon. I just love how he keeps winking and giving thumbs up to all the girls haha. The Crystal Onix episode was also pretty cool, and obviously as a kid I believed any rumour I heard that might result in me getting one in-game...
I also used to really like the Sinnoh Saga but it was the only series they used to show on my local station (don't have cable) and it would be on repeat from the first episode to the last, so I've seen every episodes about 5 times and am now totally sick of it, but that doesn't stop it from being a good series overall. I just think it has some of the stronger characters Pokemon-wise eg: Ash's Buizel, Gligar/Gliscor and Gible, Dawn's Piplup and Brock's Croagunk... Ash also has a pretty strong team (as far as the anime goes) and actually evolves his Pokemon (!!). Paul makes for a strong rival and the Sinnoh League was easily the best league (I actually liked Tobias sweeping everyone with Darkrai, more people should have Legendaries imo). Not sure if I can chose a favourite episode in the series... I liked the one with Phione, and the one when they get stuck in that tower / rocket and fly to space (Tower of Terror I think?)
So yeah, OI and Sinnoh for me.. I haven't seen any XY except for one episode this morning actually, Ash helped two brothers on a scavenger hunt or something.. I want to watch the series from the start but I haven't finished with adventures in Unova yet