From reports, it is easier to get admission in mostly some state polytechnics like Imopoly and most polytechnics in the northern part of Nigeria even Federal owned.

If you are the type that just wants admission anyhow and anywhere, straight to the north! You can get admission in a Federal Poly in the North with just 140 Jamb score. For state owned polytechnics in the north even 80 can see you through.

One of the most competitive polytechnics in Nigeria is FEDPONEK, that school is a poly that competes with Universities.

Oko poly though not too hard to get admission is less populated to compare with Fedponek. Imo poly... you can get admission there with 150 Jamb score, anything less, you are kicked out.
Here is the comprehensive list of all polytechnics in Nigeria for your referrence.
https://study-nigeria.com.ng/2018/09/01/list-of-polytechnics-in-nigeria/

If you are in any Poly or you know about any polytechnic so well, how hard do you think it is to get admission in such school?