Depends on what kind of Psychic-type you want. Overall, though, I'd suggest Reuniclus or Espeon. If you want a bulky Psychic, Reuniclus is a fantastic choice. I used one in my last White playthrough, and it was extremely enjoyable to use, as well as powerful. Psyshock at Lv.25 is fantastic, and the overall movepool is quite good, with great TM options like Shadow Ball and Focus Blast. It's also great at taking hits, but it's speed is lacking.
Espeon, on the other hand, is more of a traditional Psychic type. It's fast with high Sp.Attack and decent Sp.Defense, but not as good at taking physical hits (Though it's stats are more balanced than, say, Alakazam). However, it's still a very enjoyable Pokemon to use. It's movepool, like Reuniclus's, isn't much different from the typical Psychic-type fare, but it does get the Fairy-type Dazzling Gleam by TM, which is a very useful move. In addition, it gets Morning Sun if you want to use it.
I'd suggest one of these two Pokemon, but if you don't like either, there are other options. If you want something more physically offensive, Malamar is a weird but fun Pokemon to use (But only with Contrary), Bronzong is great if you want a full-on wall, and Beheeyem, Jynx, and Mr. Mime are fun choices if you want to use something really odd.