That's kind of asking "What's the best car?"
Older-style electronic key system or full IP-based system?
On-site switch or off-site hosted switch?
On-site voicemail or off-site hosted v/m?
Integration into an email system?
Integrate with employee's cell phones?
One incoming number & POTS line? mulitiple POTS lines? PRI trunk? IP trunk? (will you have enough low-latency IP bandwidth to support the phones? If you go IP, often the trunk and stations are provided by different companies, just like Ma Bell.)
Will you or a contractor maintain it?
Google voice has some interesting features, as do Fonality and RingCentral. There are others. I think Intermedia does phones in addition to email/web. You need to educate yourself, especially if you're opening an IT shop; I'd expect you to be advising us.