The Swachh Bharat Abhiyan (SBA), launched on October 2, 2014, by the Government of India, aimed to achieve universal sanitation coverage and eliminate open defecation by October 2, 2019, as a tribute to Mahatma Gandhi. It comprised two sub-missions: Swachh Bharat Abhiyan (Gramin) and Swachh Bharat Abhiyan (Urban). The claim that it is 'by far the most significant cleanliness campaign' holds considerable merit due to its unprecedented scale, ambition, and impact.

Reasons for its Significance:

1. Unprecedented Scale and Ambition: Unlike previous sanitation programs, SBA set an ambitious, time-bound target for the entire nation, mobilizing resources and efforts on a massive scale. It aimed to construct over 100 million toilets in rural areas and improve waste management in urban centers.
2. Focus on Behavioral Change: A key differentiator was its emphasis on changing mindsets and behaviors, not just building infrastructure. The campaign used extensive IEC (Information, Education, and Communication) activities to promote the idea of 'Swachhta' (cleanliness) as a social norm, transforming it into a 'Jan Andolan' (people's movement).
3. High-Level Political Will: The campaign received continuous, high-level political backing, which was crucial for its momentum and resource allocation. This top-down push, combined with grassroots mobilization, created a unique synergy.
4. Health and Economic Impact: Improved sanitation has direct positive impacts on public health by reducing water-borne diseases, infant mortality, and malnutrition. It also enhances dignity, especially for women, and has economic benefits through increased productivity and tourism.
5. SDG Alignment: SBA significantly contributed to India's progress towards Sustainable Development Goal 6 (Clean Water and Sanitation), demonstrating India's commitment on the global stage.
6. Comprehensive Approach: While initially focused on open defecation, it later expanded to include solid and liquid waste management, making it a more holistic cleanliness drive.

Challenges and Criticisms:

Despite its successes, SBA faced criticisms regarding the sustainability of ODF status, the actual usage of constructed toilets, the quality of construction, and the continued challenge of solid waste management, particularly in urban areas. Issues like manual scavenging, though legally banned, also persisted.

Comparison with Previous Campaigns: Earlier initiatives like the Central Rural Sanitation Programme (CRSP) and Nirmal Bharat Abhiyan (NBA) lacked the political impetus, public participation, and comprehensive approach of SBA. They often focused solely on infrastructure without adequately addressing behavioral change.

In conclusion, while challenges remain, the Swachh Bharat Abhiyan's sheer scale, its success in bringing sanitation to the forefront of national discourse, its emphasis on behavioral change, and the unprecedented political will behind it, undeniably make it the most significant cleanliness campaign undertaken by the Government of India to date. It transformed cleanliness from a peripheral issue into a national priority.