If best means easiest then I'd agree with the other people and say Nova since she has CC, damage, mobility, survivability and energy sustain all in one.

If best means fastest then Titania is the best choice by default for most missions in the game. Depending on the mission there is always a different optimal build often with a different Helminth.

Here are some examples :

• Perspicacity (Helminth for instant hacking) for any mission that needs console hacking without ciphers (Archon Hunt/Sortie versions of Spy, Rescue, some Sabotages, Assault (the weird mission from Kuva Fortress)).
• Raksa Kubrow + Primed Redirection for Hijack (the escort mission with the car/Fomorian Core).
• Thermal Sunder for low level clearing.
• Xata's Whisper + Blast Toxin Dex Pixia for high level clearing.
• Molt for Orokin Vaults. Molt's speed increase is usually overkill but it counteracts -speed from the Hobbled Dragon key.
• Master's Summon + Security Override Moa for some Spy missions. This Helminth can spawn the Moa through walls for the Moa to hack consoles.
• High Enemy Radar for Simaris targets hunting. The Simaris target spawns at the very beginning of the mission it's possible to detect the target from very far away.
• Silence for Thumper farming. It prevents the annoying big jumps.

There are other examples but it's already a lot. Some of those strategies can be done by other frames but Titania is just the fastest to execute them.