What is the best chess engine?

Mine is StockFish for many different reasons.

Strength: The strongest freely available engine overall. Stronger than top human GMs by a huge margin.
Cost: Free and open source.
Where to use: Built into many sites and GUIs (including chess.com analysis, Lichess, Arena, CuteChess, etc.).
Best for: Deep analysis, blunder-checking, opening prep, endgame study.
LCZero (Leela Chess Zero)

Strength: Neural-network engine with a very “human-like” style; extremely strong given good hardware (especially GPU).
Cost: Free and open source.
Best for: Exploring strategic ideas, long-term plans, and complex positions where human-like evaluation is helpful.
Komodo Dragon

Strength: Top-tier commercial engine, very strong and flexible (good at analysis and engine vs. engine play).
Cost: Paid.
Best for: Players who want a polished, well-supported commercial product.
If you want one engine to rely on:

Pick Stockfish. It’s free, the strongest, and easy to access on most platforms.