I have two experiences competing for top spot in my memories.
The first was Borth Bog, Aberystwyth where a friend & I sat watching a juvenile Hen Harrier & a Merlin try & catch the same Meadow Pipit for about 10 minutes; the harrier would flush the bird only for the merlin, who was perching on a nearby hide, to swoop in under the harriers talons, & invariably miss, letting the poor mipit escape into the undergrowth, only for the harrier to flush it out...
The second was on Sherkin Island, Co. Cork. I'd just finished a butterfly transect which had featured swarms of
Common Blues speckling the grass like gemstones. I sat by the sea to eat my lunch, saw a cuckoo fly over and then the clincher; an
Otter appeared about 20 feet away & proceeded to spend the next 20 minutes hunting & feeding virtually by my feet, needless to say my lunch went uneaten that day!