For the twin engine market, I would have to say the Alabeo Cessna 404 is my top choice. The 404 is a larger version of the 402. Featuring different engines, and a little different design. But it's a classic airplane that was used for private, air taxi, air cargo, and airline operations. Alabeo did a great job modeling it, and they also just released a new service pack for it. It has very few minor issues. I have about 20 hours flying the Alabeo 404. The Aersoft Twin Otter is another great add-on! It flies well, it's slower than I thought it would be for having two PT6s. But it's held at Aerosoft's quality, it has fanastic modeling and some cool features aswell! Carenado also has a wonderful Beechcraft King Air 90, and 200. The 200 is my personal favorite, I just love the 200. It is very well modeled and is easy on frames. MilViz is coming out with a King Air 350i featuring a glass cockpit and all the bells and whistles. It's still in Beta, but from my knowledge you can purchase it know, and recieve updates as it is fixed. Then Flight1 has a King Air 200 that features a glass cockpit, but I know nothing about it. Also, have heard and seen good reviews on about the RealAir Beechraft Duke, however, I've never owned or flown it.
For single engine, their are plenty of wonderful aircraft! A2A with Accusim is a wonderful add-on. Like LockheedTristar said the maintenance thing is great! It's also very well modeled. Carenado and Alabeo also feature a great line-up, I love my Carenado 206, you can upgrade it to glass. But for single engines I would recommend A2A if they have made it.
What I always do when looking into an add-on, is I look on youtube and find any kind of review of the aircraft. That way I know how it is hurting frames, or what common issues are there, things like that!