There sure were a lot of guest stars who became world leaders. I'm not talking about ones who were intially leaders but came to rule after we met 'em. Such as:

Dukat, Damar, Shakaar, Rom, Winn (World Religious Leader though), Worf (very, very briefly), Martok.

That's all I can think of. You guys have any others?

Gowron from "Reunion"-TNG

Odo, if the Founders can have a Leader, it seems like he took over at the end.

Shouldn't that be "galactic leaders"? (All right, maybe that's a little too nitpicky.)

I guess I got carried away when I mentioned Worf. But if we're going to go there why not list regulars? I'm assuming that Sisko was the leader of that world when they first crash landed on it in the 5th season. I didn't see the entire episode so I could be wrong. Wasn't there an episode of TNG where some simple aliens thought that Picard was their leader for a bit?

Yes-"Who Watches the Watchers". The Mintakins called him "The Picard".

You forgot Quark, who was the Grand Nagus in one episode.

there was that skreean woman from "sanctuary", who was deemed leader of her people for finding the wormhole.

Kirk became the leader of Sigma Iotia in "A Piece of the Action" didnt he?

The two Ferengi from TNG became leaders of that Delta Quadrant world when they fulfilled the prophesy.

On Voyager one of the guest stars became King Abdullah II of Jordan, although that probably doesn't count because it's real life.

Merat: Kirk became the leader of Sigma Iotia in "A Piece of the Action" didnt he?

Luigi Novi: I don't think so. He said the Federations was "taking over," not that he was. He was just acting as a representative.

Also:

Azetbur became Chancellor iin ST VI.

Salia was destined to become a leader in The Dauphin(TNG).

The crew told Hugh he seemed to function well as the rogue Borgs' leader at the end of Descent part II(TNG).

Proconsul Neral first appeared in Unification part I(TNG), but had become Praetor by Inter Arma Enim Silent Leges(DS9), (and played by a different actor, no less).

The Cardassian female hologram and Donik became the leader of the holograms at the end of Flesh and Blood, didn't they? (Or were they all killed?)

The crew expressed the opinion that Otrim could be a new leader for his people at the end of Friendship One(VOY).

The Skrreean woman from "Sanctuary" was Haneek.