We have had very good experiences with Delta Service Co. of Alexandria; they’re located on General Washington Drive behind the Marlo you can see from I-395 near Edsall Road. My parents have used them for as long as I can remember, at least back to the 1980s, and they recommended Delta to me when I bought my house in 2001. They replaced our HVAC and thermostat in 2014 (Carrier equipment) and they came in $4000 less than the other estimates we’d gotten (I recall one from Cropp-Metcalfe and I don’t recall who the other was). We’ve found them to be extremely reliable. I also like it that when they’re coming out here to service the system, they call to say the man is on his way. (Especially useful if I’m thinking of hopping in the shower or some such so I know not to do so.)
One potential sticking point for many people, however: Delta doesn’t take credit cards. Cash or check only, although you can use those promotional checks some card issuers send you (we did that for ours and paid it off over two months; we had a zero-fee check on which the promotional interest rate was lower than what the card rate would have been).