Once you bring in mods there's a number of options that are arguably superior to any vanilla design, but yeah, the Mantis is amazing.
And before anyone takes offense again, I don't mean it's overpowered, I simply love that ship.
For stuff that can fight back a Desdinova might work pretty well too, but she's a bit on the expensive side so whatever you're chasing better be worth it.
At this point, I can't imagine playing the game vanilla with the tiny ship variety.
The mantis is really nice, just kinda fragile. I guess the Feldspar has broken my sense of how survivable a frigate should be.
I tried a semi-vanilla setup recently, with just Nexerelin and the ships and weapons pack and it was pretty enjoyable.
Of course you're missing an awful lot of fancy designs and weapons and the sector is kinda small, but having everything fit visually and thematically was really nice for a change.
It's one of the reasons I often stay away from Scy, Neutrino and Diable Avionics (not to mention AI War and Crystanite), even though I like their stuff. They look good on their own, but the art style is noticeably different from vanilla, so if I do use them I build my fleet from just one or two factions.
Maybe that's just a mild form of OCD on my part though.
As for the Mantis being fragile, well, most frigates I like are.
She can bob and weave though, is fast, has a pretty good flux balance and allows for a lot of effective configurations.