There's a lot to choose from but here goes.
1. The multifaceted plot to retain power, obviously. Call it a failed coup, an insurrection, what ever you call it the unprecedented nature of trying to prevent the peaceful transfer of power to a newly elected prez is unique in US history.
2. Undermining faith in elections. Undermining faith in our intel agencies. Undermining faith in our bedrock institutions..........all for political gain.
3. Shattering the unwritten rules and conventions for democratic governance that are not specified in law.
What should be done in a post-Trump world to restore the rule of law?
www.lawfaremedia.org
4. The disorienting, constant chaos. Everything from the revolving door of cabinet appointees to firing Comey to refusing to cooperate with congressional investigations to causing a rise in politically motivated violence to routinely firing IG's to injecting himself in Fed policy...........etc.
5. Incompetent governance. The failed COVID response, the failure of the tax cut to achieve its stated goals, the alienation of allies, the failed trade war, failed immigration policy, failed gun policy, etc.