The big AR's, like the 9s and 90s, are awesome speakers. Plus, they don't often sell for high prices, as they are expensive to ship. They'd be my first choice, then the AR9LS and AR9LSi would come next.
Bookshelf speakers are also well covered by AR, with the coveted AR3 and AR3a. But, as an AR90 owner, I like the Vertical bookshelf models. They just seem more 'modern', and 'maybe' better built. The AR91, AR92, AR915, and AR925. All dome mid 3-ways. Plus, keep an eye out for the AR58S, a large bookshelf version of the 12" AR91, that often flies under the radar. Prices are pretty reasonable for the 58S, and they're better than the cone mid 58B version. After these, the 9LS/9LSI variations; the AR98 and AR78. Both good, but take care that combined dome tweeter/mid plate. Expensive to find used, and impossible to repair. Not fragile, just don't let little kids around them, as you don't want pushed in domes.
For a normal 2-way bookshelf, the AR18 always gets a lot of love. Nothing flashy, just good sound. Fairly cheap too to find. I'm thinking about getting a pair myself.
Oh, the ultimate AR speaker? Maybe the AR-LST. Another coveted speaker. AR designed it for studio and commercial use. Four dome mids, four dome tweeters, and the famous 12" AR woofer. Wide dispersion and awesome power handling. Good at anything. Just be prepared to pay dear for a pair. They don't go cheap. And if you ever find a cheap pair, Buy Them! Don't ask here first, if they're worth it! They'll be gone if you do.