I don't prefer one over the other. I like pretty much four types Fire, Ground, Steel, and Dark.
Fire - Partly due to starters I like this typing, I think I have choose Fire type on my first run every generation outside of Gen IV, I love their move pool and designs. I also like the newer Fire pokes introduced in Gen V Volcarona is absolutely amazing in battle and design, I love the entire Litwick like, Victini <3, then last generation Magmortar looked awesome and was quite usefully(preferred to use Mags over Infernape tbh), Heatran is awesome sauce it has some many resistances, great move pool, just good stuff.
Ground - I know Ash has an affinity for Ground types and I don't blame him, I absolutely love them. There are so many beasts among them like Garchomp and Excadrill whom are both Ubers.I generally like the designs of them as well. And my favorite or close to favorite Legend Groudon is as the name implies Ground.
Steel - OMG Steven Stone is such a boss. Some of them are hit and miss put most of them for me are hit. Like I said earlier I started every generation with a Fire type until Empoleon came about, to me it look so sleek and classy at the same time. It has a good deal of resistances, love the Water/Steel dual type.
Dark - Like with Steel there are some hits and misses here but mostly hits. It does have my favorite Pseudo Legendary in Tyranitar as well as another awesome Pseudo Legendary in Hydregion. Also has my favorite Eeveelution in Umbreon. Zoroark and Bisharp awesome looking pokes.