1. Playlist formats
The player must accept M3U links and Xtream Codes logins, ideally several playlists at once. TOS: both formats, several playlists. Learn more: M3U or Xtream Codes?
2. The TV guide (EPG)
A good guide shows what is on now and next, without slowing down. TOS: TV guide with live preview.
3. Speed with the remote
Zapping, searching, going back: everything should take a few clicks. TOS: an interface designed for the remote, and your phone as remote and keyboard.
4. Channel organisation
Hide unwanted groups, sort favourites, avoid SD/HD/4K duplicates. TOS: hide, sort, group, one entry per channel for every quality.
5. Sport
Knowing which channel shows the game is often the real challenge. TOS: a Sports page listing games with the channels of your playlist that broadcast them, and 2 or 4-channel multi-view depending on the TV.
6. Movies and series
Posters, summaries, ratings, resume playback, new episodes. TOS: full details and platform releases marked "In my list" (free TMDB key).
7. Price and trial
Beware of subscriptions that add up. A full free trial lets you check all of the above. TOS: free trial for 7 days with no credit card, then €19.99 once, 3 devices, usable for life.
What about legality?
An IPTV player is legal software: it provides no content. It is the playlist that must be legal. Read Is IPTV legal?