Elite - so units with special training and/or very long tradition of fighting.

For Poland it would be:

303 Fighter Squadron - scored more victories than any allied squadron in the Battle of Britain - its tradition begins wth American volunteer Kosciusko fighter squadron from 1920 war and one of the squadrons from the Pursuit Brigade from 1939. It indeed included most of the most experienced Polish pilots later sent to other Polish and Allied units.

Commando units in 2nd Polish Corps - fought as elite assault infantry for example at Monte Cassino.

SBS - Paratroppers Brigade trained for the support of uprising in Poland, but wasted at Arnhem at the time of the Warsaw Uprising.

'Parasol' ( Umbrella) and 'Baszta' ( Tower) -and other - Home Army elite units, uderground had their own elites as well.

1st Armoured Division - the one which fought at Falaise, in Holland and Germany in 1944-45.
It has much older tradition behind - it was elite 'Black Brigade' ( 10th motorised Cavalry Brigade) in 1939 the unit stopped 1 Panzer and 1 Light divisions for a long time and in the end was the only whole big Polish unit to retreat to Hungary/Romania. In 1940 it fought as 10th (tank) Cavalry Brigade in France and was praised by its resistence by French HQ.

In 1939 - most of Cavalry Brigades ( including 2 mechanised and 11 normal) the cavalry was elite mounted infantry thanks to their equipment ( anti-tank weaponry for example) and training, Highland 21st Division, finally 1st and 3rd Legionary Infantry Divisions.
Cavalry was especially respected by Germans - no because of some non-existent, propaganda rubbish about charging tanks which never happened - but thanks to the discipline and morale of these soldiers.
Special award in 1939 would go to the Wolynska ( Volhynian) Brigade which stopped XVIth Panzer Corps for two days skillfully using their weaponry and this way destroying numerous armoured vehicles. It retreated according to the plan and orders.

Regards Cegorach