Favorite Community Day?

The bulbasaur one would be my favorite because I've added to my collection a male normal venusaur with pretty good IVs, and a pair of shinies.
Dratini is my 2nd favorite for both the shiny and the huge amount of dratini candies. The eggs added more too during current event.
Pikachu... I already have a shiny Raichu female, but I was not able to go outside during that time so I never caught a pair for her. Maybe I'll hatch a shiny pichu some day or hunt through another pikachu nest.

I liked Dratini Day because 2 of my full grown friends finally got a Dragonite and they squealed like kids

Dratini

For me the bulba and dratini were very enjoyable, but for different reasons. Bulba's shiny is cool and Frenzy Plant is an awesome move, and it was cloudy during the event. Dratini's day was full of hype (my hunting group went nuts, me included lol) and candies i needed. It's hard to say one was better than the other. Pikachu's day (much like all pikachu based events) was very forgetful.

Good question, but I'm not sure if I can give a great answer :S

I think I'll have to divide it into three categories:
Pokemon & Move
Bonuses
Community experience

POKEMON & MOVE:
- Pikachu with Surf was a good start seeing as it's synonymous with Pokemon, but it's not a great Pokemon and we've seen a few events where Pikachu has been everywhere, so many already have the Gold badge for it. And Surf wasn't really an upgrade.

Bulbasaur was alot nicer to see and it's not without use, especially with it getting Frenzy Plant as the special move. I don't use grass Pokemon much, but I like Venosaur. So that was really nice.

Dratini is a Pokemon I don't see too often, which is why I was some way from getting my gold badge. And since Rayquaza was here at the time I really wanted gold so as to increase my chances of catching it. Dragonite is also a really good Pokemon, so getting Dratini candies and a chance at getting a good IV one (preferably a shiny one) was great. Besides, I like Pink, so the shiny version of Dratini and Dragonair is really nice. Not as keen on the green dragon, but it's alright.

Winner for me: Dratini (Bulbasaur close 2nd)

BONUSES:
- It's a bit tricky to decide here because I've been in different situations during the three Community Days. During the first one I was still chasing level 40, so double XP on everything was great for me. I could skip most of the catching and just evolve like crazy.

During the 2nd one I didn't get any extra XP, but I am also one who's always trying to reach the next million Stardust, so getting triple dust was great. That also encouraged me to spend more time catching Pokemon, which frankly is more fun that evolving for three freakin' hours.

In the recent one we had triple XP, but this time I was already at level 40, so XP wasn't as valuable for me. But I liked that it was only triple XP for catching and not evolving.

Winner for me: 1st Community Day (getting 760k that day helped me alot, getting another 3.8 million during the past double XP event got me to lvl 40)

COMMUNITY EXPERIENCE:
- Another tricky one because they've been quite similar. I've spent every one at the same place and there's always been loads of players (several hundred PoGo players at one mall, it's crazy). I could narrow it down if I only looked at the people I walked with, but I can't really say that one was better than the other.

During the first one a large group of local players met up before and after the event, which was nice, but we also walked alot together, even if some split away from the group from time to time.
The past two times most of the same players have been present, but we've just greeted eachother as we passed by.

I will say this though, I spent so much attention to my evolves during the first event that I couldn't quite do much else. It was just evolve, evolve, evolve, quick, quick, quick.
It's been much nicer to just catch Pokemon, even if that is also alot of work.

It's been fun to talk a bit more to the people I walk with and see who gets the most shiny ones etc.

Winner for me: 3rd Community Day (probably because I got a 100% shiny Bulba and the weather is much nicer).

I had hoped to have an overall winner, but it seems like each Community Day has one win each.

However, even if the large amount of XP was nice during the first C-Day, that will probably be third for me.
To pick one of the other two is much harder, but if I have to pick just one, then I guess it might be the 2nd one, simply because I like Stardust and I needed/wanted the gold Dragon badge.

Overall Winner: 2nd Community Day with Dratini!

The three were very different, for me.

Pikachu was definitely the weakest. I didn't even bother to go out for more than an hour (the last hour) but it was the only time that I caught a shiny that was a wonder (82.2%).

Bulbasaur was great. I collected a ton of stardust and evolved three Venusaur with Frenzy Plant, including one Shiny (80.0%). At the end of the day, I have less Bulbasaur candy than I started with! Because it is harder to Pineap, and I evolved three Ivysaur and three Bulbasaur. This is the only time that I was able to grind all 3 hours.

But the winner is Dratini. Not only am I sitting on a rightous amount of Dratini candy, but I managed to catch a level 31 wonder (91.1%) that became my first Dragonite with Outrage. I only pulled down a single shiny, and it wasn't a wonder, so it is still a Dratini. I evolved two total Dragonite here, so I am sitting on tons of candy to eventually power them up with.

My favorite is either dratini and bulbasaur.

Surf on Pikachu is useless, It doesn't matter if Surf is Exclusive move I don't care or want it.

Next I would be interested in Omastar with Rock throw/Rock Slide

Bulbasaur. It seems the spawn rate was higher than for Dratini, so at every stop there would be at least 3. Fortunately they rarely escaped from balls (I used Great Ball for most). In the wild they normally can be a real nuisance, just like Charmander. Furthermore Frenzy Plant is great, Surf on Pikachu not, and Draco Meteor is cool but they should have given Salamence the unique honor to use this move for a while, it is what makes obtaining him attractive.