Fire: Houndoom has been my favourite Fire critter since Gen II. I love its typing, its stats, its design, its movepool... well, everything about it! I always imagine it being like the Pokemon version of Cerberus, which is kinda cool!
Water: This one's really hard, as Water's easily my favourite type. There's Kingdra, Milotic, Lapras and Lanturn duking it out for the win... and I'd probably have to go with Lapras as the victor. I've used one in every game when I had the chance to, and I've never regretted it. As well as this, I still have memories of liking it in the anime as Ash's transportation. Maybe the nostalgia is a powerful factor.
Grass: This is the opposite of the one above, as I really really dislike Grass critters. The only one that comes close to getting my approval (ha!) is Cradily. It's slow and ugly and doesn't have a great moveset... but there's something strangely endearing about it. In any event, it's certainly much better than Armaldo!