Here is a link to an article about the species Alces alces.

Males (or "bulls") normally weigh from 380 to 700 kg (838 to 1,543 lb) and females (or "cows") typically weigh 200 to 490 kg (441 to 1,080 lb), depending on racial or clinal as well as individual age or nutritional variations.[38][39]

https://en.wikipedia.org/wiki/Moose

The word for that species in the USA and Canada is "moose", but in Eurasia they are called "elk". When I was a child I read a scene in Bambi where Bambi looked with awe upon a passing group of elk, but was surpised to see that they looked like moose in the illustration.

The elk (pl.: elk or elks; Cervus canadensis), or wapiti, is one of the largest species within the deer family, Cervidae, and one of the largest terrestrial mammals in its native range of North America and Central and East Asia. The word "elk" originally referred to the European variety of the moose, Alces alces, but was transferred to Cervus canadensis by North American colonists.

Elk have thick bodies with slender legs and short tails. They have a shoulder height of 0.75–1.5 m (2 ft 6 in – 4 ft 11 in) with a nose-to-tail length of 1.6–2.7 m (5 ft 3 in – 8 ft 10 in). Males are larger and weigh 178–497 kg (392–1,096 lb) while females weigh 171–292 kg (377–644 lb).[21]

https://en.wikipedia.org/wiki/Elk

The reindeer or caribou[a] (Rangifer tarandus)[5] is a species of deer with circumpolar distribution, native to Arctic, subarctic, tundra, boreal, and mountainous regions of Northern Europe, Siberia, and North America.2 This includes both sedentary and migratory populations. It is the only representative of the genus Rangifer. Herd size varies greatly in different geographic regions. More recent studies suggest the splitting of reindeer and caribou into six distinct species over their range.

The females (or "cows" as they are often called) usually measure 162–205 cm (64–81 in) in length and weigh 80–120 kg (180–260 lb).[146] The males (or "bulls" as they are often called) are typically larger (to an extent which varies between the different species and subspecies), measuring 180–214 cm (71–84 in) in length and usually weighing 159–182 kg (351–401 lb).[146] Exceptionally large bulls have weighed as much as 318 kg (701 lb).[146] Weight varies drastically between the seasons, with bulls losing as much as 40% of their pre-rut weight.[147]

https://en.wikipedia.org/wiki/Reindeer#Size

So perhaps they should be described by their scientific names Alces alces, Cervus canadensis, and Rangifer tarandus, so everyone knows which species is being talked about and can picture how large they are.

Other answers point out that people have sometimes ridden Alces alces and Rangifer tarandus.

This article has two photos of Cervus canadensis being ridden.

https://www.zoochat.com/community/threads/which-ungulates-are-rideable.467512/

So apparently members of all three species have sometimes been ridden.

I give no advice about which would be the best - or least bad - to be domesticated and ridden.