There’s no single gsm termination provider that has the best quality and pricing. Quality has a direct proportion with price (the better the quality, the higher the price). But I’ll try to explain what factors have influence on price and quality.
Top destinations / routes
Usually each provider specializes on a specific destination or a region. When you approach a carrier, ask what their top destinations are. Because they send a higher traffic to those destinations, they get volume discounts. As a result they can offer a lower price to their clients.
Tiers / levels
There are tier 1, 2 and 3 providers. Tier 1 are final call terminators, tier 2 buy from tier 1 and tier 3 buy from tier 2. The more tier there are in the call chain, the lower the quality is. However, it’s important to note that tier 1 won’t be willing to work with small providers.
Route type (direct, premium, grey, etc.)
The best quality routes are direct routes. but they cost the most. The cheapest are grey routes that are done by terminating traffic using VoIP-GSM gateways. But grey route quality can be terrible. If you’re looking for something in between you can search for mixed/blended routes.