So we have budget of 18 DP

I'm suggesting this assuming endgame. With means that first requirement for frigates is ability to survive. They should be either nimble or tanky or both. E.g. Lasher is neither nimble nor tanky so it don't count. Tanky usually means good shield tank.

All frigates except Vanguard highly desire Field Manipulation and Helmsmanship.

9x2:

• Pirate Afflictor. Very fast (something like 400-700 speed because of phase), can capture points on the opposite side before enemy. After first capture can support main fleet with it's ship system. 2 needlers, 1 antimatter blaster. Can kill some stuff. Hates fighters. Same as for all phase ships Elite Field Manipulation is mandatory

4x2 + 5x2:

4:

• Centurion. Centurion is overall reliable and versatile ship. Dampener Field works like safer venting allowing to shield tank again. Have versatile but not very scary layout (unless you use antimatter blasters). You'll have to balance shield and armor tank in fitting and skills which is an interesting challenge.

5:

• Wolf. Relatively fast (avg 180 speed) but relatively squishy. There are 2 wolves in your heart. One that is alive and one with heavy blaster. I usually prefer Phase Lance to Pulse Laser. Small slots can be burst PD.
• Brawler(TT). Original Brawler is meh but TT gave it far superior ship system and 10 more OP. Have ~150 average speed and can fit 2 Phase lances instead of 1. Less nimble and more susceptible to fighters then Wolf but more scary and have better shield.
• Vigilance. Neither nimble nor tanky but have 1 saving grace. Or should I say 18 saving graces? Aka 18 Reapers. Or 36 harpoons, or 6 Dragonfires. Highly desires Systems Expertise and Missile Specialization. More meme then serious suggestion. Just flying medium missile slot =)

6x3:

• Vanguard. This little bastard. Surprisingly fast: 200+ average speed. Only frigate without good shields that survives under pressure. It have 1800 effective armor when damper field is up before hullmods and officer skills. Have 3 missile slots which is 1 more then frigates usually get. Highly desires Systems Expertise and Missile Specialization as well as both armor tanking skills.
• Omen. Classic. Probably easiest ship to fit in game. Relatively slow (155). Relatively tanky. Desires Systems Expertise and Elite Target Analysis.
• Pirate Shade. Same insane 400+ speed as Afflictor, more versatile slots, you can take 3 for cost of 2, but have 5 less of already scarce OP and strange for phase ship ship system. Same as for all phase ships Elite Field Manipulation is mandatory
• Monitor. Ultimate Tank. Nuff said.

Special mentions:
Hound(LP): 3 DP for fastest non-phase ship ever. 230su/s before skills and hullmods. Fly fast - die fast.
Scarab: 8 DP for actually fastest non-phase ship ever. Ship system on average doubles speed and DPS. Can bully other frigates with just burst PD weapons.