I guess if you're new in Sofia and just want to use prepaid service you can go with any of the carriers. 

Mtel lost a lot of customers back in 2010 when they failed to implement a new billing system and people received ridiculously high bills with a huge delay. This wouldn't affect a prepaid client but was very annoying for everyone on a monthly plan.

So this is when they lost my trust and I switched to Vivacom, keeping the number (as others above have suggested, porting your number is easy now in BG). So far I'm happy with the bills I'm paying. 

Years ago I used to be signed up to Globul and in my opinion their customer service was awful. According to some of my friends it still is. So I guess my recommendation would go for Vivacom :)