The T20I matches of 2018 have already started. So we now look back at the matches of 2017. Not too many as there was no world championship, though there is now an increasing trend of 3-match series rather than a single match grudgingly tagged on at the end of the season.
First, the ICC ranking table at the end of the year:

This has a close bunching at the top, with 3 points separating Pakistan, New Zealand and India.
Now we look at win-loss ratios in 2017:

Pakistan and Scotland (!) at the top, with Bangladesh just above bottom-ranked PNG. Although Pakistan does top both lists, it should be remembered that the ICC rankings cover a longer period.
Now we look at individual performances. With a relatively small sample size there is not much point in finding averages and strike rates. The matches involving the World XI are not counted, even though the ICC and Cricinfo regards them as official T20Is
Batting: Most runs (150 and above):

So Evin Lewis of the West Indies leads, leaving established stars like ABD and Kohli behind. C Munro is the only one with 2 centuries, while several have three 50+ scores.
Highest innings (75 and above):

Evin Lewis leads the next in line RG Sharma by a large margin (by T20I standards).
Bowling: Most wickets (10 and above):

YS Chahal emerged with a bang, while Williams and Rashid also emerged from the fringes. Chahal is the only one with three 4wis, no one else had more than 1.
Best innings bowling (all instances of 5wi and above):

Chahal and Rashid Khan again.
Fielding: Most dismissals (8 and more):

Dhoni heads this list, although surprisingly Ireland’s GH Dockrell has the most catches as a non-keeper. In fact he has more catches than Dhoni or any other keeper!
Best innings fielding (3 or more dismissals):

While Chandimal, Dhoni and GD Philips have 4 dismissals apiece, several non-keepers have taken 3 catches.
All-round performance-overall:
See criteria below:

Seekuge Prasanna emerges as the only to one to have bowled and batted enough with some pretensions to being an all-rounder.
All-round match performances (20+ runs and 2+ wickets).

The best performances here are by Anshuman Rath (HK) and Shakib Al Hasan.