In order of likeness, for me, it goes:
1: Gold/Silver/Crystal
2: Diamond/Pearl(/Platinum)
3: Ruby/Sapphire/Emerald
=: Red/Blue/Yellow
GSC brought about the first time (granted, they had only done two generations to start with) and the only time you could actually go to more than just the designated Region the game was based upon. That was something I really liked it for and hoped to see in the next generations, as well.
GSC was also, until D/P came, the only other generation to have any form of Eevee evolution within its "new" ranks, and even brought daytime and night time to accommodate nocturnal and diurnal Pokemon types, such as Hoothoot and Pidgey, respectively. (correct me if I'm wrong, but I think Pidgey didn't turn up at night)
It also brought about Day Care, which allowed you to breed Pokemon; in some occasions, to acquire a "baby" Pokemon, such as Pichu and Cleffa, while in others, just to get another Pokemon of the same species. This would also be the only Generation where you can talk to your Pokemon, while they're in the Day Care; others have cordoned off sections, where identifying your Pokemon is pretty much impossible.
D/P brought some good themes and two new Eevee evolutions. It put a new perspective on things, and made the game brilliant with concepts quite similar to PokeGear and PokeNav of Gen 2 and Gen 3, respectively. Although the Poketch didn't allow you to make calls, like the PokeGear and PokeNav (latter only in Emerald) did, it didn't really matter, because it had and gained very helpful apps for people to use, as they play the games; such as a clock and a Day Care Checker.
It also brought back day and night; the physical presence of day and night, which R/S/E feigned on, by only having clocks; even at night, it seemed to be midday, on R/S/E.. one major problem with D/P is that there are only two Fire types, and one of them isn't that amazing; it was effectively Nintendo saying "Use Chimchar, or go through the game without a Fire type." It also brought back my liking for Water type starters, from RSE's failed attempt at keeping me with them.
RBY were the first Pokemon games released on Ninntendo's hand held, and it was a great way to bring it in. Looking back at the games gives you that nostalgic feeling, and for its time, it was a great game; not to mention the only one we really had, until Stadium came into the works.
Anyways, RSE are down in (joint) third because it was the weakest Generation, for me. sure, they had some good things in it, like the PokeNav and brought about Contests for the first time, ever, but it had a few personal weak qualities, from the start.
One was that I actually preferred the Fire type, over the Water type, in starters, which hadn't happened through the first two Generations (meaning I hate Mudkipz. Yes I do. =] ) and the second was that it was a more water-based region, which put a downer on the Fire type, because it really limited how you could fight. -Before you quote the above written, please do remember they are personal qualities that I found to be weak. If you don't agree, that's fine; just don't flip out about them.- One good thing is Gen 3 brought another set of dragons for the currently smaller base of dragons, from the first two Generations, bringing three Legendary Dragons and a band of non-Legendary dragons, including the likes of Altaria and Salamence.
I'll not say I didn't enjoy RSE, because I did, but it does stand as one of the weakest Generations, in my opinion.
