Which tasbih is best?

If you mean which strand to buy

The best strand is the one that matches how you actually count, and after that it is preference.

No material makes a dhikr more accepted. A strand from Makkah counts as a plastic one from a corner shop. Anyone pricing beads on spiritual grounds rather than on the stone is selling you something that is not theirs to sell.

• Count 33 after salah and nothing longer — a 33-bead strand.
• Want the whole post-salah sequence in one round — a 99-bead strand with two feelable dividers.
• Count in the hundreds — a counter will serve you better than any strand.
• Use it around others in a quiet room — wood or plastic, not glass or stone.
• Want it to last — check the cord, not the beads. The cord is what fails.

If you mean beads, a counter or fingers

The method with explicit evidence behind it is the fingers. Abdullah ibn Amr said he saw the Messenger of Allah counting the tasbih on his fingers (Sunan Abi Dawud 1502, graded sahih by al-Albani).

Scholars differ about the strand itself. Egypt's Dar al-Ifta holds bead strands recommended. IslamWeb and Ibn 'Uthaymin hold them permissible with the fingers better. Al-Albani held them an innovation. Al-Suyuti and al-Laknawi wrote treatises defending them.

Every one of those positions condemns using a counting object to be seen. The comparison in full is on tasbih counter vs beads.

If you mean which dhikr is most powerful

This is the more common intent behind the question, and it needs a careful answer rather than a ranking.

The texts do not present dhikr as a league table. Several phrases have specific virtues narrated for them, and those narrations describe different things — some speak of forgiveness, some of weight on the scale, some of what outweighs a long session. Reading those as a ranking, and then declaring one phrase the most powerful, goes beyond what any of them say.

What can be said accurately is which phrases carry a narrated count and which do not, because that is a fact about the text rather than a judgement about reward.

• Astaghfirullah — 100 a day — Sahih Muslim 2702.
• SubhanAllahi wa bihamdihi — 100 — Sahih al-Bukhari 6405.
• Juwayriyah's phrase — 3 — Sahih Muslim 2726a.
• The two heavy words — no count narrated — Sahih al-Bukhari 6682.
• La hawla wa la quwwata illa billah — no count narrated — Sahih al-Bukhari 6384.

Why "most powerful" is the wrong frame

Two things go wrong when the question is asked this way.

The first is that it turns dhikr into optimisation — finding the phrase with the best return per repetition. That is not how any of the narrations present it, and it changes the act into something transactional.

The second is practical: it is the frame under which invented virtues spread. "Say this 313 times for wealth" and similar claims travel precisely because people are looking for the most powerful option. A number of those claims are examined on 15 tasbih claims that are wrong.

The more useful question is which phrase you will actually say consistently, with attention, and understanding what it means.

The short version

For the object: buy the one that fits your count, check the cord, ignore anyone pricing beads spiritually.

For the dhikr: the established phrases with sound references are on what to say on a tasbih, each with its source. Pick from those, keep to whatever number is narrated where one is, and do not adopt a count you saw on an image.