Gen I I agree was pretty drab environmentally, but the pokemon designs were (mostly) solid. A few were somewhat left-field, like Jynx, Mr. Mime and the two sludge pokemon Grimer and Muk, but by and large I didn't object to most of them. It isn't my least favourite generation, and I tend to let off the simplistic environments due to it being the first one on the most basic hardware.

Honestly? It's a toss up between Gen III or Gen V for me. I found the legendary designs for Gen III pretty crap. Kyorgre and Groudon looked very computer generated and I'm just not a fan of their designs whatsoever. Beyond the legendary design, it also introduced the Pokemon contests which I didn't very much care for. They also took out the day and night cycles in Gen III, for some reason, despite two eeveelutions depending on them.

As for Gen V, again, the pokemon designs. Now most of them weren't bad, but there were a few really questionable ones. I'll mention the obligatory ice cream cone pokemon. Every generation has its fair share of bad designs, but my patience starts to wear. Not only do they have less pokemon to design, they should just generally be getting better practice. That's more of a gripe than anything else though. It certainly doesn't 'ruin' the generation for me.

Black and White 1 also attempted to reboot the series so to speak by making it all new pokemon until after the Elite Four. A nice idea, but I remember not having many fire types on my team, or some of the rarer types around. It also meant if you're not a fan of the designs you're forced with them the entire main game. That, and they made the final evolution of the fire starter Fire/Fighting AGAIN for the third time in a row, which got tiresome fast. I never picked Tepig, basically.

Overall? I'd probably say Gen III. This isn't to say I didn't enjoy both of them. They're good games. I was just least enamoured with these two. Gen V did introduce reusable TMs though, which was a godsend.