That depends on what i want to use it for. For an 'all-rounder', i go for nuvi 5x0 which is a multimode waterproof unit with removable li-ion battery. It can be used as an auto unit for car or bike, an outdoor handheld for hiking and can function in boating mode if a marine map is loaded. It can be flashed to zumo 2xx to get full motorcycle functions. It's 'nuvi' name is a misnomer, seemingly labeling it as an auto-only device. I keep a nuvi 500 in the car with a spare battery for emergency use. 5x0 was never a popular series with General Public [who should be demoted to Private for their lack of wisdom and finesse] and so they tend to be not easy to get hold of 2nd hand now. The ppl who have them realise their value and hang on to them i reckon.
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For something more 'modern' for auto use only, my first choice would be the nuvi 34x0 and 35x0 which can be mucked around with to add various functions including video & mp3 player, and junior 37x0 can also be played with to enable DEM and 3D Buildings and Landmarks, or in the case of 3760 [which has BT but not ASR], it can be tricked up to add ASR effectively making it a 3790. 37x0 can also be flashed to 34x0. Junior 1xxx can be flashed with 14x0 logo to enable extra features so that'd have to make the list too.

It's all just personal choice really and dependent on what you want to use the unit for, eventual practical day-to-day use or the fun of modding. Garmin has got really crafty with the latest units and made their modding quite hard [read impossible for the likes of me]. Drew a big blank even with Loly's help trying to add vid/mp3 to 3597 for instance, can mod rgn140 but the app icons don't appear in the selection like with 34/35x0. If you want to do something monumental mod 3597 firmware to allow effective manipulation of region 140 to show the icons.

PS: Had to have a bit of a at this:
Quote Originally Posted by osiris4isis View Post
... If you were to save up to get one, which one? .......
I've got well over 50 units, mostly auto devices. Haven't counted them for a while, might be 60+ by now .....