Why we dislike the Sinnoh dex?
THREE Water/Ground types
THREE Water/Flying types
ONE Fire type (Not accounting the starter)
A LOT of completely useless stuff (Mothim, Pachirisu, etc etc)
A LOT of pokes that were too hard to count for the results they yield (Wormadam? Vespiquen? Please...)
That might help.
I'm way too lazy to look up comparison numbers for all the other gens, but
1. Water/Ground is fantastic typing. Both are useful types, and combined they're only better. Having three of them gives you a choice. Having several Water/Ground types is nowhere near as bad as Gen 1's pure Poison-types who were all basically useless.
2. 3 Water/Flying types? Okay, got me there, but Wingull and later Pelipper, combined with Bibarel, made the premier duo of HM slaves until Platinum's easier-to-obtain Bibarel and Tropius. And Gyarados just destroys shit and shouldn't really be a score against the Dex because it's in every single one.
3. Like I said, yes, there was one Fire-type besides Chimchar's line, but it's available at the point where it's usefulness is high and early in the game. There was only one, but it was placed in the perfect spot. Platinum fixed this also by adding some more for choice.
4. >Implying there aren't a ton of useless things in every other dex ever.
5. This goes with the useless Pokemon bullet, and seperating it is really just trying to manufacture another reason Sinnoh's dex Pokemon aren't that great. If they're so hard to get, just ignore Honey Trees. Sure, you won't get Heracross and Munchlax but there are still many good Pokemon choices.
Sinnoh introduced a lot of Pokemon who were excellent in the archetype, like Staraptor, Luxray, and even Bibarel was not awful like, say, Furret. One of the things the Sinnoh Dex has done better than any game so far is distribution of useful Pokemon. Many decent Pokemon are available during the first half of the badges in the game, when most people are building their team because any later and it's hard for the new Pokemon to keep up, which most games fail at doing. Platinum only improved, with LOLGiblebythirdbadge and more Fire-types and new evolutions to buff up some of the new Dex additions that weren't available until after the E4 in DP.
The only bad part about the dex, I'd think, is that a Pokemon is either shit or great for beating the game, and since people of course would want to use good stuff, there's less variety for building a decent team for the storyline. Other dexes tend to be mostly shit in my opinion, balancing things out. And I'm not sure that paragraph makes any sense, just trying to think of something I genuinely didn't like about the Sinnoh Dex. I'd really argue it was the best one.