The only way to answer this objectively is to agree on a single measure of poker skill. I doubt it this is possible.
Nevertheless, here are a few possible measures:
- Most $ earned. Flawed because of difference in stakes and tournament fields.
- Most BB/100 earned in cash games.
- Most finishes in the money in MTTs
- Most first places in MTTs
- Highest win rate in MTTs and SNGs
- Most diverse player(cash games, tourneys, different poker games)
- Most beautiful player(for females)
- Most respected by fellow pros
- what else? ...
My personal favorites are Doyle and Phil Ivey, but there are so many outstanding players, it is a really tough call.
And I don't know much about Stu Ungar except that he is considered one of the greatest players by pros. Maybe we should start doing drugs as well.