Let's take a look at Lunatone. Base 70 HP, 55 Attack, 65 Defense, 95 Special Attack, 85 Special Defense and 70 Speed. Those stats aren't terrible, but they really aren't very good either. While it does get plenty of good moves (Ice beam, Psychic, Shadow Ball, Signal Beam and Calm Mind for example) I'd still recommend using a different Rock or Psychic type Pokémon. Here are some I'd recommend:
Gigalith. There are a few ways to obtain this one. There's somebody on Route 7 who will trade you a Gigalith in exchange for an Emolga (there's also a chance of finding Emolga in the bubbling spots on Route 7!), or you could catch a Boldore in Chargestone Cave and evolve it. It has a very high attack stat at 135 and a defense that's at 130. It also has 85 HP and 80 Special defense which is pretty decent. Though this one doesn't get many good moves.
Aggron. You can find Aron in Mistralton cave. Aggron has a massive 180 Defense and a good 110 Attack. Its other stats aren't very good though. Aggron resists a bunch of types due to being half Steel, but its 4x weak to Fighting and Ground. Aggron could also make a decent HM slave if you really need one because it gets Cut, Surf and Strength. (not sure why you'd make it a HM slave when it has massive attack but I thought I'd say that anyway)
Beheeyem. You can find Elgyem in the Celestial Tower. I used a Beheeyem in my playthrough of Black and she was an absolute monster. It has a great 125 Special Attack stat, a nice 95 Special Defense and decent other stats (75 HP, attack and defense, though it has a terrible speed stat at 40.) It gets a few good moves like Energy Ball, Thunderbolt, Shadow Ball and Psychic (it also gets Calm Mind to set up with).
Starmie. Staryu is found on Route 13 by Surfing. Starmie is very fast with its 115 Speed and also has 100 Special Attack so it can deal a lot of damage. Its defences aren't bad either (it has 70 in each). Starmie can also learn Surf which is good if you don't already have a Water type in your party. Starmie doesn't learn many moves via level up (it only gets Confuse Ray at level 22) and Staryu also doesn't get many good moves via level up. So while Starmie is a great Pokémon, you'd need a few TMs unless you wanna be stuck with stuff like Swift and Brine.
I hope I helped out!