dave2008
Legend
For 6 people at 17th level you would need a solo monster with a CR about lvl +10 and per the epic encounter guidelines you could need up to a CR 30 solo monster. So, here are your "official" options (from DnD Beyond):Running out of MM options! On the upside the players aint that tactically brilliant.
At the moment I really need a non-boss not-smart single monster of large or huge size.
30 Aspect of Bahamut Fizban's Treasury Of Dragons
30 Aspect of Tiamat Fizban's Treasury Of Dragons
27 Black Greatwyrm Fizban's Treasury Of Dragons
27 Blue Greatwyrm Fizban's Treasury Of Dragons
28 Brass Greatwyrm Fizban's Treasury Of Dragons
28 Bronze Greatwyrm Fizban's Treasury Of Dragons
28 Copper Greatwyrm Fizban's Treasury Of Dragons
28 Gold Greatwyrm Fizban's Treasury Of Dragons
27 Green Greatwyrm Fizban's Treasury Of Dragons
28 Rak Tulkhesh Eberron: Rising From The Last War
27 Red Greatwyrm Fizban's Treasury Of Dragons
28 Silver Greatwyrm Fizban's Treasury Of Dragons
28 Sul Khatesh Eberron: Rising From The Last War
30 Tarrasque Basic Rules
30 Tiamat Rise Of Tiamat
27 White Greatwyrm Fizban's Treasury Of Dragons
Not a wide range as the greatwyrms on actually only 2 stat blocks. Now, I have created some high CR monsters of my own. Here is partial list of the high CR monsters I have that might work for you:
CR 27:
Black Dragon, Wyrm (Legendary)
Black Dragon, Great Wyrm (Elite)
Empyrean (Legendary)
Hezrou Horde (Legendary)
Kord (Legendary)
Warforged Juggernaut (Elite)
CR 28:
Demogorgon (Elite)*
Green Dragon, Wyrm (Legendary)
Green Dragon, Great Wyrm (Elite)
Zakum (Legendary)
CR 29:
Blue Dragon, Wyrm (Legendary)
Blue Dragon, Great Wyrm (Elite)
Dragotha (Elite)
Genius Loci (Legendary)
Gigante (Elite)
CR 30:
Desert Leviathan (Elite)
Phaethon (Legendary)
Red Dragon, Wyrm (Legendary)
Red Dragon, Great Wyrm (Elite)
Tarrasque, Primordial (Elite)
CR 32:
Hecatoncheires (Elite)
Tiamat (Elite)
*Elite is the LevelUp version of "Mythic"