@bitter linden Hi, I've done a fair amount of live performing and competing, my 2 cents:
I understand the desire to save your best songs for last round but think of it this way, if everyone else uses their best songs in the first round and beat you, you'll never make it to the last round anyways. Start strong because every round could be your last.
That being said, you want songs that showcase technicality as well as an ability to emote with your voice. Technical abilities you should be trying to showcase in rough order of importance:
- Pitch accuracy
- Tone
- Musicality (phrasing, emotiveness)
- Breath Control (dynamics, held notes)
- Power (controlled volume)
- Vocal Agility (arpeggiation, runs)
Note that vocal agility is last on the list. While crazy runs can generate "wow" moments and be flashy and impressive, if you don't have everything else down pat the runs won't do you much good.
By that same dint you should** avoid** songs that have simple/repetitive melodies or limited vocal range.
1 more thing to add is that it is not impossible but it is much harder to pull off a winning performance with a sad or slow song than one that is upbeat and energetic. If all you do is stand and sing you won't get far. Remember it's a talent show and honestly the show part is more important than the talent part.
Hope this helps.