Q:

which is the most frequent formulation of the Gods of the Trimurti?

Brahma-Vishnu-Siva
Brahma-Vishnu-Mahesh
are the ones I have seen the most.
Any others?
Is this order the most frequent because Birth-Preservation-Annihilation is the natural order?
note:  this is NOT a venue for "my God is superior to your God" type answers.
from Encyclopedia Britanica:

Trimurti, (Sanskrit: “three forms”) in Hinduism, triad of the three gods Brahma, Vishnu, and Shiva. The concept was known at least by the time of Kalidasa’s poem Kumarasambhava (“Birth of the War God”; c. 4th–5th century ce).
The trimurti collapses the three gods into a single form with three faces. Each god is in charge of one aspect of creation, with Brahma as creator, Vishnu as preserver, and Shiva as destroyer. In combining the three deities in this way, however, the doctrine elides the fact that Vishnu is not merely a preserver and Shiva is not merely a destroyer. Moreover, while Vishnu and Shiva are widely worshipped in India, very few temples are dedicated to Brahma, who is expressly said to have lost his worshippers as the result of telling a lie and is merely entrusted with the task of creation under the direction of one of the other two gods. Scholars consider the doctrine of the trimurti to be an attempt to reconcile different approaches to the divine with each other and with the philosophical doctrine of ultimate reality (brahman).

A:

The most frequent order, when talking about the three Gods together, is Brahma-Vishnu-Maheswara and that is due to the fact that creation (with which Brahma is associated) comes first then preservation (linked with Vishnu) and then destruction (associated with Mahesha).

1-7. The Risis said :-- “O highly fortunate one! A great doubt has
  arisen on your statement. This is ascertained by all the wise men as
  written in the Vedas, Purânas and other Sâstras that Brahmâ, Visnu
  and Mahes'var, these three Devas are eternal. None is superior to
  them in this Brahmânda. Brahmâ creates all the beings, Visnu
  preserves and Mahes'var destroys all in due time. These are the causes
  of creation, preservation and destruction. The Trinity Brahmâ, Visnu
  and Mahes'a are really one form, indeed, Trinity in Unity and Unity in
  Trinity.
Devi Bhagavatam Book 1, Chapter 8

And, the order is also evident from the following hugely popular verse (from the Guru Stotram found in Viswasara Tantram):

गुरुर्ब्रह्मा गुरुर्विष्णुर्गुरुर्देवो महेश्वरः । गुरुरेव परं
  ब्रह्म तस्मै श्रीगुरवे नमः ॥१॥
Gurur-Brahmaa Gurur-Vissnnur-Gururdevo Maheshvarah | Gurure[-I]va
  Param Brahma Tasmai Shrii-Gurave Namah ||1||
Meaning:
  1.1: The Guru is Brahma, the Guru is Vishnu, the Guru Deva is Maheswara (Shiva),
  1.2: The Guru is Verily the Para-Brahman (Supreme Brahman); Salutations to that Guru.

